Hmmm... 1st if you dont want to use gelcoat, just use the f/glass resin itself, thicken it with a filler something like silica flour is perfect or indusrial talc, there are a few things to thicken it with then just apply this to the surface let it go off hard but before it cures completely (meaning the surface is still a little tacky)lay it up now with the f/glass matting. As for using f/glass on plaster it can be done yes but not directely on top of the plaster ( plaster must be dry as water retards the curing of poleyester resin) you have to seal it 1st. Plaster is porous so you need to have a smooth surface, to do this you can varnish the surface with shellac, about 6-8 coats should do it, when ready wax and polish this surface then lay your f/glass into it as suggested above, I hope this helps you in some way.
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