Wednesday, May 25, 2011

More thoughts on channels

Putting channels in the board is definitely on the radar for board #3 but I'm still working out the best approach.

The approaches that I'm most excited about at the moment are the double bagging idea ( a full bag around the board and then half bag to clamp the bag onto the rocker table) and the idea of building the board upside down.

The double bagging idea, I think, would work for boards where the majority of the surface area of the bottom side of the board is not channelled. My thinking here is that area that does not have channels on it will be be in contact with the rocker table surface and so the entire force due to the vacuum on the upper surface will need to be supported by the non-channeled area. This means that the pressure on the the non-channeled areas will be subject to multiple atmospheres of pressure rather than just c. 75% of an atmosphere when the entire bottom surface is in contact with the rocker table.

The resin that I work with says that you should be careful when applying pressures greater than 1/2 an atmosphere as it can cause the lower molecular weight components to separate from the resin mixture and weaken the final product. In the case the pressure at the contact points would be many atomspheres pressure and , at least for foam cores, might be enough to squash the peaks in the channels.

One possible thing that might help ( thinking out loud here) is to build the core up out of several layers of material and do it on the rocker table so that the core already has much of the shape held in place by the internal stresses of the core material layers before you try to laminate it. This would reduce the clamping pressure by the second (1/2) bag if using the double bagging approach. Alternatively, maybe there is a way to use steam and pressure to put most of the rocker and concave in the board prior to laminating.

In production boards they get around this by having 3-d moulds for the top and bottom surface that match perfectly with the contours of the board so that the pressure is distributed evenly. Other than being able  keep the pieces cut out from the channels to use to create the 3-d surface on the rocker table I can't really see how achieve this. Maybe it would be possible to make up pieces close to the size and shape of the channels and cover it with compressible material so that under pressure the load could be distributed more evenly. Perhaps the pieces could be cut from PVC foam and use the channels themselves with sandpaper laid in them to then shape to PVC foam to exactly match the channel channels.....

The other idea of building the board inverted (upper deck on the rocker table) has some real appeal. It would put a pristine finish on the upper side of the board straight off the rocker table. It would allow narrower boards to be built as the step down from the top layer to the typically recessed rail reduces the width available for the footpads to sit without there being overhang and it would allow the vacuum bagging to be used to apply a uniform pressure over the contoured bottom side of the board.

The downside is that contouring on the top would have to be very limited unless 3-d mould 'inserts' could be created and it would mean that it may not be possible to use the plexiglass as the rocker material as it would be difficult to get the plexiglass to bend into the concave of the table surface. However, alternate surfaces might work. MDF painted with high temp paint and clearcote might work as might self-adhesive plastic film (mylar for example) stuck to the surface. Using thin metal sheet at the table surface might also be an option. Thin sheet metal might also be good ( I'm guessing) for using in heat curing although I wonder if it might buckle?

My production board CrazyFly raptor has no contouring on the top surface and I really like look of it so for me this upside down idea is pretty attractive.

Saturday, May 21, 2011

Constructing Channels

I've been thinking more about how to laminate the channels.

From thing I've read on the web one approach production houses take it to have top and bottom molds that simultanously apply pressure to the contoured surfaces of the boards with the laminate in place. In one case I read that up to 90 tonnes of pressure is applied an 20 minutes later the board is ready to drill and clean up.

Obviously not an option for home but the dual mold idea might have a poor second cousin that could be DIY'd.

The idea would be to use two rocker tables (or if you've got an adjustable table two different settings) so that you could first lay up the underside with the board upside down on the 'top' rocker table and vac bag it so the channels get laminated properly, then laminate the top side with the board vacuumed onto the bottom rocker table. You could use this second step to put a smooth finish on the bottom at the same time as laying up the top.

My non-adjustable 'brokite style' rocker table already has the jigs cut for the top rocker table as they just the excess material from cutting lower jigs. All that would be needed would be to screw the rocker surface center jig for the top rocker table.

If the board has very little profiling then it might be possible to do this in a one step process. If the top of the board is profiled then without support under the profiled pieces they would be bent out of shape e.g the rails might end up being bent upwards (which might actually not be a bad thing). I wonder if it would be possible to to cut inserts to support the profiled sections that don't leave to creases in the laminate? Maybe there is a way to use a hot wire cutter to profile the foam core? This seems like it would be a fairly high level of precision which might now be possible to do in the backyard.

Another idea, would be to do the profiling on the underside of the board. This would mean tapering the tips on the underside rather than the topside. This might work. You would just need to take this into account when calculating the rocker and concave and would need to make sure that the tapering was smooth rather than the typical well defined wedge that comes about using a router. The manual CNC ideas that Jorge Gonzales put me on to on would work perfectly here. (  BTW check out Jorge's pro-setup . Awesome!). This way the board could be layed up upside down and you could end up with the really slick finish on the top that you get on the underside when you use perspex or the like on an bottom rocker table. 

The more I think about it the more I think this way could work well. 

You could also use the channels to help control the flex by either carving them into the core for more flex or adding extra material to build up the channels for less flex.... This might just work.......


Friday, May 20, 2011

Viagra Leak Detector!!!

A call out to Harry Nespolon for contributing the kiteboard builiding movement by donating a special edition Viagra stethoscope to use for detecting leaks in the vac bag!!!

Special Edition Viagra Stethoscope for detecting 'hard' to find leaks:)

Wednesday, May 18, 2011

Board #2 Debrief

Before it starts to fads from memory I wanted to run through for my own learning what worked and what didn't with board #2.

Core preparation

The good

i) inserts: using the flange nuts pushed through ABS washers with the washers having 1mm holes drilled through it were a great replacement for the t-nuts. The small holes let the air and resin flow through to the top so the insert can be moved easily to the right right spot. I drilled the insert holes all the way through the board this time and filled the bottom with about 3mm of 5 min epoxy. When I set the inserts in place I put a bolt all the way through the insert and sat it on top of the 5 min epoxy. In board #1 there wasn't the 3mm layer before the laminate and so when I overtightened the screw it pushed through to the fibreglass and delaminated it.

Also, pouring the resin very slowly into the insert hole let the resin push the air out ahead of it so I had much less air trapped in the resin.

ii) Using a really thick q-cell (microballons) mix to put a radius on the join between the upper layer and the lower layer. The mix was really like peanut butter. I had used much thinner mix in the board #1 and a lot of it flowed away and I had to keep reworking until it set.

iii) rails: Cutting the ABS in straight strips (10mm) and then heating and forming it around the shape of the core worked very well. Last time I tried to cut the ABS to the shape of the board and wasted a lot of material and didn't get a good mating of the rails and core.

iv) Not gluing the long sides of the balsa veneer together. This lead to needing less clamping pressure to get the concave in and as the gaps open very slightly when the concave was added it allowed the resin to get right into the groove.

v) Sanding: I found that sanding with sandpaper tapped to long planks of wood helped get long smooth curves. I found that small sanding block lead to the board edge being a bit wavy.

The bad

Air bubbles
I still struggled to keep air bubbles out of the q-cell / resin mix. I had tried to mix it more slowly as I'd read that vigour mixing can put air in the mix that ends up getting trapped. The bubble issure reduced but was still more like an aerobar that I wanted. I primarily used it in the inserts. I used q-cell here to reduce the weight and to ensure the inserts themselves where not visible. i think that next time I'll just use pigment to obsure inserts. I don't think that the weight saving is material enough to warrant putting the strength at risk.

Gluing the rails to the core - The low viscosity epoxy resin easily flowed away from the gaps created by the slight mis-match between the core and the rail. The heated forming of the rails meant the mis-match was small and I used some q-cell in to thicken it up and improve its vertical hold. Still it flowed away and pooled under the board. Solution would be to put tape on the underside and rail to hold it in place and create prevent the resin flowing away (der!) Pouring the rails seems to all round be a better solution - no adhesion issues no shaping issues.

Laminating flat - with the balsa wood being stiffer, the final core was significantly stiffer that than the klegecell alone. I suspect this may have lead to some more spring back in the final board than board #1. Laminating it all on the rocker table might have been a better idea so that some of the shape is all ready held in.

Balsa veneer - the balsa veneer looks great and I'm going to make more this way - it just looks very specy.  However, laminating with just a thin (1mm) veneer created more work than was probably necessary as I needed to glue the spars around the outside of the top layer to that the foam core wasnt' visible. This meant a long wait time again before proceeding. I think that next time I might use 5mm balsa as the top layer rather than 2 layers of foam and balsa veneer. This will speed the process up but will make for a slightly heavier baord.

Rocker table

Go the perspex!!! Straight off the table the bottom finish was awsome. I didn't even need to clear coat it (but probably will for protection) to get a mirror finish. Also, tacky tap for vac bagging had no problems sealing to it. One tip with tacky tap - when removing the vac film rip it of in one hard quick motion. It will come off without stretching the vac film or tape so it can be reused. Didn't need much mould release to get the resin to separate from it. I suspect you may not need any but why run the risk.

Board-off Design Tool. Being able to print the design templates for the rocker table meant I got the rocker and concave spot on.

Sanding the surfaces of the side jigs on the rocker table at an angle so that the rocker surface when it bends down to sit between then the surfaces of the jigs are parallel to the rocker table surface. If the aren't flush with the the bent surface the amount of rocker and concave clamped into the surface will be wrong as there will be a significant gap between the top and bottom jigs.


The good

i) Mixing the right amount of resin (according to the datasheets) so that you don't end up using too much resin. Not only does it leave a mess on the rocker table and the board but it will weaken the board if excess resin remains. This is because more of the internal stresses will be borne by the resin (as there is more of it) and resin is about 30 times weaker than e-glass. Hence it breaks sooner. I followed the 50:50 ratio by weight and added an extra 10% for resin left in the cup and excess need to ensure it saturated well. I wasted so little resin this time and using the right amount more than likely reduced the potential for resin pooling under the peel ply.

ii) Use proper peel ply - in board #1 I used ripstop nylon left over from an old kite. I put a lot of holes in it to help the resin flow through it to the bleeder material and thought that the weave of the material would be enough to avoid resin pooling under it. However, the combination of too much resin and the nylon having too little porosity meant the resin pooled under the nylon and left the surface of the board very bumpy. Presumably I could have sanded it back but that seemed like a lot of hard work.

This time I used proper peel ply which wicks the excess resin away and apart from the imprint of the weave left an very uniform surface - no big lumps. Its cheap about $5 a meter. Well worth it.

iii) Used proper vac film - I've been told that the benefit of vac film is that it stretched around sharp corners. I fyou have a relatively flat piece you're laminating then you can use standard old builder plastic that you can buy from bunnings. I found some builder plastic but it was only marginally cheaper and had grit embedded in it. Not sure if there is any advantage to it. The vac film, which is polyethylene plastic is strong and uniform so I can  see any good reason to substitute.

The bad

i)_Air bubbles in the laminate. I am starting to believe that some air bubbles are inevitable and require some post cure work to cover and fill them. I worked the resin a hell of a lot harder with the squeegee this time but the tiny air bubbles trapped in the bottom layer only reduced a little bit. I have been told to use a roller ( aluminium type) and a lot of elbow grease - apparently this is called consolidation. I have also seen videos of people dabbing down on the laminate with the bristles of a chopped of paint brush to burst the bubbles and force resin in. You'd need to work quickly to use this approach so maybe the roller is a better idea.

ii) To get a smooth finish on the board to replace the impression of the woven peel ply on the surface, I re-coated it with a small amount of resin and vac bagged it with the vac material directly on the surface. While this did leave the surface with more shine on it, surface was not uniform and had the impressions of wrinkles on it. Overall it was a disappointing finish. Next time I think I'll resign myself to needing to work the surface post curing. sanding, clearcoating and polishing. this seems unavoidable unless protective layer like PBT is being used. However, the PBT is expensive ( I've only come across a supplier in the US and its about $80 printed and delivered.) and apparently adds weight as it traps the resin.

iii) Reusing vac film and tacky tape - as occurred in the first board when I reused the vac film and tacky tape I got leaks that I couldn't close off. Finding the leaks is a real  bastard. I had fairly rapid leaks at one point without the tell tail hissing sign that normally goes along with it. So, I had to just keep working around the edges and pushing it down. I have a feeling that threads of glass falling across the tape may have been responsible for keep small channels of air open. The only way to tell would be to have some really sensitive listening device. I'm going to see if I can get hold of a disused 'stethescope' or whether there might be Jaycar electronics kit for such a thing. I have a vague memory of seeing something like it.

Tuesday, May 17, 2011

Channels, what the?

There has been a good discussion going on on seabreeze around the impact of channels on a twin tips performance.

The concensus seems to be that channels are about increase the grip of the board and is especially noticable when your fins a small or riding finless. It seems to me that they act like very long thin fins so that their action is felt along the full lenght of the board rather than just at the tips.

From the board analysis work I've been doing they wouls not seem to contribute greatly to the stiffness of the board although if the channels are built up from stiff material is may help. However, this would seem to be secondary to the flow control it provides.

It seems to make sense that the smaller radius channels ( smaller than the radius of the single concave) would mean that the angle that the bottom layer enters the water when edging will be greater and hence the increased grip. However, from my limited experience if you don't have sufficient rocker in the board,this stepper angle of attack will force the board up and out of the water and you'll loose your edge.

I have also read about the lossy effect of cross flow. This is where the water flows across the board instead of down the length of the board so that less lift is generated by the flow. I'm guessing that the channles might also redirect the flow a bit and help improve lift.

So how to add channels to the bottom of a DIY board?

I haven't got my head around this one yet. Single concaves are fairly easy with the aid of a solid rocker table. However, the channels add a new level of challenge. In production boards I believe they use precise aluminium molds that have surface contours in place and then they load them up with up to 90 tonnes pressure for short periods ( 20 mins) so everything gets squeezed together nicely. The DIY rocker table is more limited to say the least.

Thinking out loud, some of the approached might be:

i) lay the baord up with minus the last bottom layer of glass so that the rocker and concave are largerly held in the board. Cast of carve the channel profiles in a seperate piece of material ( foam, balsa or just build up from scap glass and resin) then laminate the seperate channels under the last bottom layer

ii) Double bagging. Carve or build the channel profiles into the core prior to adding any laminate. Add laminate to the bottom of the board with the board on a flat surface.Put the flat board in a vac bag and suck out the air. Then place the core facedown on the rocker table and cover with a second vac bag and vac. clamp the baord down to the rocker table to introduce the rocker and the concave.

iii) Create the core by laminating very thing 2-3 mm layers but do so on the rocker table so that the adhesive between the layers puts the rocker and concave into the board prior to putting the laminate on. Then you could put the laminate on without the need for clamping it to the rocker table. The issue with this one would be the the spring back of the core in the absence of the exterior laminate.

iv) Carve the entire board out of a single piece of wood using a CNC machine so that there is no need for a rocker table. CNC machine would be need for this one.

v) .......

Sunday, May 15, 2011

New blog for the theory side

Because this blog seems much better off being used to track the trial and error of my construction projects, I've decided to kick off another blog to keep a record of the theory that I've picked up in the course of reading up on material needed to understand the flex profile of the board.

For the moment I'm going to use the new blog to document the beam theory and theory behind composite materials that I've used to model the flex profile with the goal being to highlight the importatn relationships among the key board parameters and the flex profile.

Once the model is sufficiently developed the next practical step will be to create some test strips to calibrate and verify the model.

Once I get a few more boards under my belt and get some feedback from others I'm hoping to be able to investigate other parts of the boards handling but for the moment I'm keep my aspirations focused on flex.

The new blog is at


Tuesday, May 10, 2011

Flex modelling - whats the point?

BoardOff Design Tool has had a great response with about 30 downloads so far!

Thanks to some comments from users on another the use/value of the flexmodel tab is becoming clearer.

The flexmodel tab in BoardOff is an attempt to model the flex of the board when bent at the middle and at the tip under different loads. The theory needed to do this is well developed in the theory of composites and the theory of beams and in many cases further simplifying assumptions can be made that are theoretically justifiable.
Q:  So whats standing in the way of the model accurately predicting the flex in a board in advance of building it?
A: the big variation in the properties of the materials you use ii) small parameter variations due to construction variations and errors to which the flex is highly sensitive e.g core thickness, and concave, resin modulus variation due to ambient temperature.

So, if there is so many uncertainties in the material properties what is the value of attempting to model it if its predictive power for the next board is limited? I believe that the real value lies in the ability to quantify what impact changing a parameter will have on the flex of the board and hence which parameters the flex is most sensitive to 'all else being equal'. While it may not be possible to guarantee that your next board will flex 10cm at the tip under a 20kg point load, you will be able answer questions like:

'All else being equal'....
i) if I increase the concave by 1mm what would that do to the flex?
ii) If I use 10mm core instead of a 12mm how much will the flex change?
iii) To increase the flex should I remove a layer of glass of reduce the core width given the accuracy of my setup?
iv) What difference will tapered tips make compared to a step profile in the tips?
v) If I change the orientation or type of reinforcement I use how will that affect the flex?
vi) What sized paulonia stringer would I need to end up with the same flex I would get using the 'carbon stringers' like the Bro-kite masters?

These can all be read off the model to allow you to develop rules of thumb by which to build the boards. For example the models show that the flexural rigidity of a foam core board at a given point is is proportional to the core thickness squared and linearly dependant on the thickness of the laminate. So taking 1 layer of glass off a 3 layer layup ( each side) will reduce the flexural rigidity by 33% (for equal weight and orientations of reinforcement) where as changing the core thickness by 10% changes the flexural rigidity by 21%. One 'rule of thumb' to come out of this is that for each 1.5mm of core thickness you take off you may need to add 1 extra layer of glass based on your previous laminate schedule.

These rules of thumb are what typically develop with the board builders experience but this modelling exercise helps to speed the process up and deepens understanding of why production boards are built the way they are and may challenge some wives tales.

The modelling exercise is far from complete with the model primarily being for flex affects. However, two of the other quantities of interest are the maximum stress and strain in the board. The holy grail of this model is to predict strength. Unfortunately this is very difficult and I suspect the rules of thumb will have to incorporate big margins of safety.

The situation with tensile strength is not as bad as it is with compressive strength. Tensile strength is less sensitive to variations in construction than compressive strength is. The reason being that failure under compression is rarely because the compressive limits of the material are reached but more often that local imperfections in the laminate or bonding layer of core surface etc lead to concentrations of compressive forces and delamination of the laminate. In this way the compressive limits are rarely reached before the board breaks. The challenge is further increased by the 'micro failures' in the laminate that accumulate over time and change the properties.

The first step in this process is simply to see whether the magnitude of the forces predicted by the model are of the right order of magnitude and if not, discover why not. Then ideally you would create some test pieces with all sorts of variations in materials construction technique and do destructive tests on them. From this one could create a set of margins of safety that could be applied to design decisions.

Finally, and for me most importantly, it absolutely fascinating to learn about this stuff. The fact that its applied to kiteboards is all the better but being fascinating is almost enough in its own right.

Saturday, May 7, 2011

Finally its final

Finally I'm declaring the board finished. This final stage of finishing the surface took twice as long as the rest of the construction because I was trying to get the finish on the board as slick as possible and in the process took a couple of steps backwards.

Re-coating the board with epoxy and vac bagging it with the film directly on the board surface did make the surface very shinny ( got rid of the peel ply imprint) but it left the surface a bit wrinkly where the film wasn't tight enough to be pulled flat. I'm thinking next time I'll just leave the peel ply imprint on the board (which actually looks really good in its own right but in this case clouded up the view of the wood grain) or try the other idea of placing a piece of 3-4mm perspex over the top of the vac film when doing a second, finishing bagging. If this can make the surface look like the bottom which came off the table looking shop then it will be worth the expense. One adjustment to the design that this will require it to not have step downs from the top surface to the rails. The board will need to be fairly uniform thickness across the width so that the perspex can conform to it.

Anyhow, the final product is shit loads better than my first attempt and now I know more about what does and doesn't work so hopefully next board will be another step up.

So here's some pictures of the final product!!

Board #2 with all the hardware installed including the shit hot CrazyFly 2011 footpads.

Out of focus but shows the quality of finish that was possible using the perspex rocker table surface. No work was needed on the bottom to make it look shop quality.

Close up of the surface finish that resulted from placing vac film directly on board. Notice the wrinkles introduced!

Me doing an ape impersonation to test the strength. A very nervous moment! However 75kg in mid section and a bit of flex but not as much as it looks cause 4.5cm of it was already there. 
Overall its been a big step forward compared to the first attempt but there is still a lot that can be improved. Next post I'm going to do a bit of a debrief of what worked what didn't and what I'm going to try next time.

Thursday, May 5, 2011

Finishing off the board

I used peel ply on the top surface of the board and because of its weave it leaves an imprint on the surface of the board that gives it a mat finish. It also obscures visibility of air bubbles in the resin which are pretty likely to be there.

The most common method seems to be to sand it back and apply another coat of resin to get the shiney finish.

A quick search on turned up, as usual, some other great tips. Seems like the best results come from having the vac film directly against the surface of the board. An even better result can be achieved by placing 3-4mm plastic perpex over the top of that and then a second vac film over the top to smooth out the surface.

The risk of not using peel ply or perforated release film is that excess resin will be trapped under the film and the pools of resin will leave the surface lumpy and add unnecesary extra weight. I think this is what happened in my first board where I used ripstop nylon as peel ply. Appartently one way to mitigate this is to squeegee out the excess resin as the vacuum gets applied.

To finish this board off I decided to put a thin coat of resin on. I mixed up about 50 gm of resin and put it in the microwave for 2x15 seconds to thin it out. I then dabbed the resin with the end of a paint brush over the entire surface to get the resin into any air bubbles on the surface then vac bagged it. I put some rolled up rags around the outside of the board so as the vacuum increased the plasic was pulled tigh across the surface of the baord. At the same time I squeegeed as much of the trapped air away as I could while the vacuum increased.

A thin layer of resin and back under the vac film to get a shinny finish on the board.

Monday, May 2, 2011

BoardOff Design Tool delivers its first board - get a copy.

This board was designed entirely using BoardOff Design Tool I put together in MS Excel 2007. It lets you design the outline of the board using cubic splines, the rocker table center and outer jigs ( for continuous rocker only at the moment) taking account of the rocker table width, estimates the flex profile (beta at the moment) and takes a stab at the weight.

You can have a free copy of v1.3.3 by clicking on the link below. It has been designed on MS Excel 2007 and is a macro enabled xls file (.xlsm). I have used third party macros in it for the splines and a beta version of DXF export so scan it with your virus software if you're concerned.

Download the latest version here

If you find it useful or have any suggestions it would be great to hear from you.

In the end I'd like to get some folks with development skills involved and turn this into an open source desktop application ..... anyone, anyone???

Off the table

I left the board on the rocker table for another 12 hours to let the resin completely cure. The board came off the perspex no problems at all ( one question answered). Overall it came out very nicely!

It weighted in at 1.85kg!!! Which at first seemed to light and I was worried that I had squeezed out too much resin as I was expecting it to come in at around 2.1kg again. However, I added a tab to BoardOff to calculate the exact area and hence the weight of the laminate and it turns out that the area of baord is just 0.47m^2 and so the laminate all up should have been around 1kg for a 50% fibre fraction and not the 1.3-1.4 kg I estamated as this included the excess glass and resin.

So here's some photos
Fresh off the rocker table

The bottom is ultra smooth having been pressed on the pristine perspex table surface.

Target concave of 7mm came in at 6mm but I suspect it may relax a bit more over the next few days. Look at that shine!!!

Before working the rails back. The extra rocker is very noticeable
Overall I'm very happy with how it came out. I reckon the first board was about a 4/10 and I'd rate this one about 7/10. Its hard to get close up shots with the iPhone so I can't show the details of the things that don't look so flash but the main thing that I still haven't been able to crack is getting rid of tiny air bubbles.

The air bubble problem is only marginally less than on the first board despite working the resin a hell of a lot more this time. There aren't the large areas of trapped air that I had last time but the pin hole sized bubbles are just as pervasive on both the top and the bottom.

I got a tip from a friend who is an experience boat builder who said to do another coat of resin with a paint brush and dab the resin into the holes. I think I'll do that on both the top and the bottom and then re-bag it.

The peel ply wicked away a fair bit of resin but leaves a very clear imprint of the weave ( which I had been told). Apparently this is great if you are putting another layer on because the surface area is dramatically increased. However it also means that there is no gloss on the finish. So I might kill two birds with one stone and put another think layer of epoxy on top and vacuum bag it with the vac film directly on the board so that it will come off glossy.

Sunday, May 1, 2011

Top Laminate goes on

Learning from my mistakes this morning I just finished laying up the top laminate with full compliment peel ply, bleeder and breather material.

I bought some proper peel ply instead of using the perforated rip stop nylon that I used in the first board. As soon as the peel ply went on it started wicking the resin up and within about a minute was almost completely wet out - presumably because there was excess resin around around on the top. The jury is still out on the ripstop nylon. It was a must stronger barrier to the resin flow than the peel ply which combined with the excessive amount of resin I used last time may have lead to pools of resin being trapped under it and leaving the surface uneven. I'm not entirely sure but this seems like the most likely culprit.

I put down a layer of plain cotton material over the top (which was originally going to be color material for bottom of the board) to soak up more of the excess and still covered the whole thing in paper towels and rags to keep the air channel open. Last board I used polyester wadding that is used for filling inside quilts. It seemed to work well and but the density of it was not uniform and so I had a suspicion that this may have contributed to the uneven finish on the previous board. Again not sure.

I spent a lot more time working the resin than I did on the previous board. A lot more pressure on the squeegee and worked it until all the crackling/popping that is apparently due to air bubbles being pushed out/popped. Around some of the tighter curves I used a paint brush with the bristles cut down so the bristles were very stiff and dabbed this over any area where it looked like there might have been air trapped. Once the vac film starting to clamp I ran my fingers around the rail/sidewall joins because last time this was were a lot of resin accumulated.
With 400gm resin and under 25inHg small amount squeezed out shows the resin/fibre mix is about right.

I replaced the vac film even though it looked like it was still in good condition. Last board I reused it and after about 2 hours I was still not able to seal off all the leaks and this resulted in the vac pump running continuously for about 10 hours rather than the few seconds every 10-15 minutes that possible with a better seal.

I clamped it under 25 inHG for a few minutes as I find that taking it way up in the first instance helps it self-seal off some of the small leaks at the tacky tap. Then I back it off a bit and leave it around 20 inHg. 

Curing away under the car port - Working in cooler temperatures has made for a less hurried process.