Can I pour liquid polyurethane casting resin directly over an epoxy tooling gel coat as a much faster way to make a split composite mould than by building up layers of fibreglass and resin as normal? It is a small mould so cost is not an issue.

I would suspect the bond would not be perfect.  I wouldn't want to be pouring polyurethane over wet/tacky epoxy so you'd have to wait until its hard and although it would stick, wether it would stick well enough to keep the gelcoat intact is another matter.

You've also got exotherm to deal with and differing heat expansion which may ruin the gelcoat.

If you want a quick and easy mould, you would be better off using our Epoxy Mould Making Putty which is designed to work with epoxy tooling gelcoat.

Otherwise stick to the traditional layup of multiple layers of woven glass to build mould thickness.


Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Thanks for your reply. I have subsequently found some information from an Australian who has successfully done this using epoxy mixed with aluminium trihydroxide filler over a tooling epoxy. It was suggested that polyurethane casting resin would probably also work but had not been tried. The biggest consideration was avoidance of excessive exotherm due to the deep moulding by using a very slow curing catalyst.