I'm about to start the repair of some fibre glass car panels so that I can ultimately pull moulds from them.  All the advice for finishing says use a 2pk filler primer, which is fine, but there seem to be differing opinions on which 'flavour', epoxy, pu or polyester.  I'd have thought polyester was a non starter for the same reasons your advised not to have polyester filler exposed in your buck/pattern?