Two layers plus foam and another layer
Graham,
I have typically tried to stay with two layers of 5-6 oz then 1/8" foam in areas where there are larger flat surfaces. Put a sheet of plastic on this and many little bags filled with sand to make the foam conform to the surface. Knife slits also OK in the foam to get it to conform. Once cured overnight remove the bags and the plastic and feather the foam edges with coarse sandpaper or a knife or both. Then one more layer of cloth. Two or three more layers in areas where you are doing fasteners.
Instead of gelcoat I wax the mold minimum 8 layers of wax then with a rattle can gray primer i lightly coat the inside of the mold. Almost dusty spray layers at first . Let them dry. Then go with a few heavier wet layers of the same primer. I have tested a spot with a finger nail to make sure the paint is sitting on top of the wax. Critical!. Then lay up the cloth and epoxy. The epoxy will stick great to the primer. Pull the part, dewax it and then proceed to wet sand with water with a bit of dish soap in it. As soon as you get to the epoxy stop sanding, and apply more primer if needed.
If this sounds scary do a test bit first to ensure you will release.
Steve
Microspheres in epoxy as a 'Gelcoat'
this afternoon while sanding a piece of bodywork I made years ago i was pleased at how well the microsphere filled epoxy repair sands and works. I will try mixing some (wearing a mask of course) in a spreadable consistency to see if I can paint it onto a waxed mold surface as an epoxy based gelcoat substitute. I have never successfully painted straight epoxy onto a mold. It doesn't wet out into a sheet like gelcoat does. My idea would be to get the microsphere mix painted into the mold and as soon as it is stable enough start laying up the cloth have at it. Let the entire epoxy mass cure simultaneously and not leave pinholes
Steve