[metal99]

Properly catalyzed polyester resin will only stay in liquid form for a limited time. I have some special polyester resin that is used for vacuum moulding and it has a one hour work time. If you under catalyze the resin sure you can get it to harden if you bake it but it won't have the same properties of properly cartelized resin. They are not meant to be heat cured like your doing but hey if it works for you then giver!

You might be able to get the proper thinner at a fiberglass shop. I know my father in laws shop has it. That's where I got my slow cure polyester resin from. It's also thinner then your normal polyester resin.