use a rairly heavy mat near the last coat, and a finer mat to finish, it doesn't print through as easily.
Should actualy have the mold/core as smooth as possible.
Test your foam with the resin alone, most of the time the resin doesn't effect the foam, for a permanant core/mold you want the resin to soak into the foam's pores for a mechanical bond, that way it will never 'fall' out. if you plan to remove the mold, you want it to be as smooth as possible and heavily waxed. The reason your foam lost it's sponginess is because the resin impregnated the foam, and bonded to it, which if you want it as a permanant core is what it should do. (spray foam is very porous)
If you want it as a mold where you pop off the finished project, you want to coat the foam with something like plaster, bondo, or tons of varnish coats, wax it up good, then lay your fiberglass over that. when you get all the layers you want on it, work the fiberglass off with wooden pry bars etc.
If you use styrofoam and carve it to shape, apply a coat of plaster of paris to that, sand smooth and varnish/wax etc, then lay the fiberglass on it. use gasoline to dissolve the foam, then a rubber hammer to break up the plaster and pop it loose by smacking the front of the fiberglass, then take a putty knife and gring it so it's rounded instead of square and scrape off whatever didn't pop off.
if you want the fiberglass to lay down as smooth as possible, lay it out on the mold dry, and stretch or cut any wrinkles out, cutting it in the best shape to lay as flast as possible first, then use "T" headed straight pins from a fabric store and pin the glass in place, then apply the resin to the cloth, when it just starts to set, pull the pins out, that avoids lumps from thick cloth folds and wrinkles, and makes it a LOT easier to start your first couple layers, after them, if you used the righ laminating resin, the old coats will still be tacky and hold the next layers in place untill set, laminating resin doesn't totaly dry in air for a LONG time, they sell a wax to mix in on your final layer to sheild it from the air so it will fully cure. .
Bondo is polyester resin with fillers mixed in, fiberglass is polyester resin (cheaper) or an epoxy base (not cheap but much stronger than poly) Gel coat is polyester resin mixed with talc (and dye when colored) check west marine's website, they have a lot of info on fiberglass through links etc (have a 29ft sailboat, so have become pretty familiar with west marine and boaters world etc.)
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