Quote Originally Posted by rspears View Post
If you've got a good fit with the plastic lid, and if you can accept that the fiberglass piece is going to be slightly bigger since it's laid on top of the fitting part, then simply wax the plastic with a good coat of wax and lay the glass mat on top. Let it cook off for an hour or so and it will pop off easily and you can then sand it smooth, hit it with a coat of filler and it's done.
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..