I am making a plug and have a particular projected finished size. How thick should I make to plug undersize to allow for two layers of low weight fibreglass cloth and resin? I may err on the side of caution and make a tad smaller than I’d otherwise expect and finish with car body filler. It’d need filler to smooth out anyway
Your plug needs to be identical to the piece you would like to make.
Also keep in mind that vynil ester and poly shrink anywhere from 1 to 3 percent (on the y axis). Not sure what kind of accuracy you shooting for.