Quote Originally Posted by lamin8r View Post
There is no reason,for a fiberglass piece to be larger,Roger,,since a mold is usually taken off the outside surface of that part..If you take a mold from that part,,what comes out of the mold will be the same..... Unless,of course,that the plug is taken out of the mold,,before the mold has finished curing,,in which case,,the mold will shrink slightly..And,,it is very hard to take a mold off a thin plastic item,,or,,as I call it,,a ''street weight'' fiber item..They need to be braced well,,and build the mold slowly,,a couple of layers of glass at a time.. AND,,,DON'T use too much gelcoat in the mold,,as if it's too thick,,it will shrink from the mold as it dries..
For a small item,,I will make my mold about 3mm thick,,built up over a couple of days,and a week of curing..minimum..
See, there's the reason for the question as you jumped right to taking the part that fits and making a mold of it, while my inexperience pointed me to simply using the piece that fit as the mold, making a fiberglass over lay of the part, popping it loose and then working it to finish. Your method is right, taking time to make a mold from the part, built up slowly and maybe even sticking on some ribs for bracing it straight, and then being able to make multiple identical parts out of that mold with a gel goat finish, too.