some foams are sesistant to resin, but all in all, the foam, or whatever you use for the "plug" is protected from the first layers by a film of wax (muli- layered), then, PVA...... then i'd spray up a layer of tooling gel, and fianally a lthin layer of resin/veil. then you can slowly build the mould up with resin/mat, i use vinyl-esther resin and thin uni-directional chopped strand matt, once the mould gets to be about 3/8" thick, ill build it up a little quicker.....

you have to be carefull not to go too fast as the mold will warp as the heat from the resin/catalyst reaction isnt dissapated slow enough...... (does that make sense?)

the most important thing in beginning the mold is to MAKE SURE the plug is waxed and PVA'D, or the tooling gel will, or can bond, permanantly with the plug..... then it will never release.

also, splits in the mold must be determined before the initial layup as to facilitate the pulling of parts from the finished mold, you cant pull from a negative draft.