I got some great help from XSWind on kiteforum re pre-cure topsheets. Here's the link
... and here's the guts of it.
The purpose is to obtain a thin skin which is resin as that is the only way to obtain a full and deep gloss/sheen.
The topsheet is wetted out on just a sheet of normal glazing glass and then vac bagged flat, there is a or two trick here but I will leave you to discover them to make it perfect.
yes I vac bag mine.
The objective is to make a cured and finished thin laminate.
Thickness is up to you but I normally do a 2oz or thinner scrim layer with decals next followed by a layer of 4oz. Leave to sure then bond
If done right you end up with a completely finished fully glossy outter surface to your board.
Once the topsheet is cured leave thay Maylar if you like sheet attached, dont remove it.
skuff the top sheets bonding side with 40 grit and clean the dust off. Now simply go to your rocker table lay up the normal structural layers and put the pre-cured top sheet on and after the normal vag bagging process has been done, you can remove that film layer to reveal the goodness.
Another tip is "a simple addition to the pile before bagging"