At this point we don't know if it is epoxy resin or polyester resin. Personally I have never seen polyester resin go bad. Its hardener MEKP can and does go bad. Being that the resin has gone cloudy I'm going to assume it is epoxy. I agree with Tom that any additives must be added in after the epoxy/hardener have been mixed well. Acetone is usually not used for thinning epoxies. Usually 98% Isopropyl Alcohol is. It's always best to cure any epoxy at 70 degrees or slightly higher.