I was assured that the PVC foam won't collapse under 1 atmosphere pressure ( the maximum this set up can pull) so the issue is what pressure will squeeze out too much resin and level the board weak. I couldn't find anything definitive on Google and I just guessed that 20 in HG (around 10 psi) seemed like a reasonable - time will tell if it was too much.
Before trying it with glass etc I wanted to do a test run to make sure all the equipment worked that there was no leaks in the system.
The Test Run
For the test run I put the board on top of the polyester wadding ( from Lincraft) that will be used as breather cloth over the board when doing it for real. Otherwise, the set up was identical to how it would be when going live.
So I set up the rocker table and applied a few coats of the release agent (basically carnuba wax like used for surfboard wax). The jar recommends 5-6 coats with 1/2 hr in between to let it set. I just wanted to keep the process moving so I started before the dry run.
I cut the vacuum film to size and cut a hole through in the corner closest to the pump so that I could connect the vacuum fitting for connecting the vacuum line. Install the connector.
I then ran a line of tacky tape all the way around the perimeter of mold surface and made sure the board was going to fit. Tacky tape is basically a strip of stretchy tar and it sticks to itself and whatever else so leave the backing paper on until the last minute. I also found that the tape could damage the vacuum film when I pulled it off after the trial. The trick to getting it off with out damage is to rip the plastic off hard and very quickly. Otherwise the tape will stretch and stay stuck to the plastic.
Tack the vacuum film to the ends of one end of the mold then pull the vacuum film over the mold and tack it to the other end, again by the extremities. Do not stretch the film as but to it so that the tension just takes the large wrinkles. then I exposed all the tacky tape and
Now the moment of truth. Will the setup hold the required level of vacuum???
Great sigh of relief as the vacuum pressure dropped to 20 inHg and with a bit of tuning of the wing nut on the regulator got it to cut out right on pressure.
I let it sit for an hour and it held the pressure for about 20mins before the pump reengaged for all of a few seconds and maintained the pressure spot on.
A couple of things that the test run showed up:
i) the larger diameter PVC tubing that I had for connecting the vacuum connect (10mm compared to the 8 mm that is used everywhere else in the setup) could not stand up to the pressure drop and over the space of about 20 mins it collapsed down. In a live situation where it needs to hold this pressure for hours, it is possible the the tube could pinch off and isolate the pressure under the film from the pressure in the pump and so cause the regulator to no sense the pressure increases it needs to respond to. the solution that ended up working fine was to get about 5-6 lengths of wire and put it inside the tube to ensure the collapsing tube can't pinch off. By contrast the 8mm had no problems at all.
ii) Not an issue but just an observation, the vacuum film was able to stretch around most of the sharp corner created by join between the top and bottom layer. However, it was not complete. Right in the corner it bridged the 2 perpendicular faces with a corner of radius 2-3 mm.
Final Amendment to the Core
I started to get nervous about how much was the right amount of resin and coupled with some confusion about how the excess resin that I was inevitably going to use was going to escape I decided to drill some 1mm holes through the core to allow it to escape if need be. I put the holes about each 5-6cm across and long the board. As it turned out some of the excess resin did push up and into the breather material so it seems like it was a good idea.Also, as I mentioned earlier I mixed up about a 50% by volume mixture of resin (and hardener) with qcell until it was thick like Vegemite and used this to fill out the sharp corner where the top an bottom layer join so that the glass would not bridge this and weaken the board.
Glass the Bottom
Before you start mixing the resin make sure that your vacuum film is cut to size and the connector is in place.
So here goes. My first attempt at fibre glassing.
I decided to do just the bottom in case I stuffed things up I didn't want to waste all the resin and glass. As it turned out this was a good thing as it made it very clear that I had used too much resin, confirmed that the ripstop nylon did its job brilliantly as a peel ply replacement and confirmed that the vacuum pump was up to the job of holding the clamping pressure for 8 hours.
I finished off adding an extra couple of coats of the release wax and let it dry.
I cut out the 3 layers of 6oz cloth I was going to use and cut it to size with about 4-5cm extra all around. 2 of the layers were cut with the weave running off axis as much as the 90cm width of the cloth would allow. Its not the recommended 45 degrees as the cloth was too narrow. I think it came in at around 30 degrees off the long axis. Apparently this helps with overall strength....
I mixed 750gms of resin as your mate at FGI said about 230gms of resin per layer (about 1/2 sq. m each layer). However, I forgot to take into account the 5:1 ratio of resin to hardener and so ended up with 900gm of resin+ hardener and so ended up with about 250gm of valuable resin sitting in the bottom of the container after I'd drenched everything with the mix. Even with this much left over, so about 650gms of resin, in the cloth, I still had too much. So my feeling it that I should have used about 200gms per layer (1/2 sq m layer).
Lay out each layer of glass, pour about 200gms of resin by running it from one end to the other and work it around with a flat edge of a something like a credit card. Make sure there is enough resin on every part to let the cloth just turn translucent. Any more that this is not necessary and will just be squeezed out anyhow.
Lay down the core and put the ripstop nylon ( used instead of peel ply) and polyester wadding over the top of the core. The idea of the nylon is that it is a woven material and so, under pressure, will allow the excess resin to be wicked away up through the weave and into the breather material. The beauty of the ripstop nylon which was salvaged from an old kite is that it won't absorb the resin and so wont remove any resin in its own right.
The breather material (also called bleeder material) keeps a channel open to allow the air to be sucked out evenly over the surface of the core.
Below is a picture of the layered setup prior to going under the vacuum film.
So now to cover it with the vacuum film and seal it. Run line of tacky tape across both short ends of the table. Tack the vacuum film on one end first and then the other making sure to apply a small amount of even tension to the film to avoid getting wrinkles in it.
Then stick a wade of breather material under the connector so that it remains open and make sure that this material drapes of the breather material covering the board as you need to maintain an open channel for evacuating the air.
Then run the lines of tacky tape down the long edges.
Then apply the vacuum.
Below is a before and after of the vacuum pump in action.
If the pump gets the air out and the seal is good then it just a waiting game. If the pump runs continuously or never gets over a certain level then there is a leak somewhere and that really sucks because it can be a bastard to find them. Firstly try finding an wrinkles in the plastic that have been pushed into the tacky tape. Fine wrinkles can let enough air in to undo your pumps good work. Try stretching them out and pressing then harder into the tape. Sometimes the air rushing in is audible - a great help. The worst case is that there is a pin prick hole in there somewhere.
I found one hole that have been caused by the film being pulled down over a small piece of gravel that ended up on the table somehow. A wade of tacky tape did the trick.
Below is a shot of some of the excess resin I was referring to above being squeezed out the end and wicked up into the breather material. Amazingly, the separating action of the nylon meant that all this excess resin went into the breather material an almost none was left on the top surface of the board.
The photo below shows you the resin that rose up through the holes I drilled in the board.
Its a good idea to sand these with some 50 grit sandpaper before the top layers go on so that you get a good mechanical bond between the dry and wet resin.
So after 10 hours on the table, time for the great unveiling. .......
When I took the board of the table I was amazed at how little stiffness there was in the board when it had the bottom on it. I was a bit stiffer than the foam but still bent under its own weight. As Andres at Balmoral Boards pointed out its not until the top layer goes on that yo turn this single layer of fibre glass into the 'I' beam that has all the strength that composite structure delivers. And want a difference it made when the top when on. It went from bending under its own weight, to being able to support me standing in the middle of it while the tips spanned between 2 chairs.
Before you do the top layer make sure you fill the footstrap inserts ( the t-nuts) with candle wax to avoid resin going inside. Wax is a good idea as you can just hit it with a match and get rid once you've finished glassing the top and you drill out the holes.
I repeated the process to lay down the top layers of glass and this time mixed up 600gms total ( 500gms resin 100gms hardener) and this was just enough ( probably another 50 gms would have been a good idea).
Cloth down, resin down, work it around and in and get rid of all air bubbles in each layer before the next one goes on.ripstop nylon, breather cloth, vacuum film. Suck the air out.