I've used both denatured alcohol and acetone for thinning epoxy and I keep both in store for just that purpose. I never use more than a couple drops to thin it, we're not talking 1 part hardener to 1 part resin to 1 part alcohol/acetone mix. We're talking just a few drops. As for heating epoxy? Not only does it shorten the pot life but the additional fumes emitted from heated epoxy are worse than the acetone even in a reasonably ventilated area.

The best idea here is the one using the finishing resins. John Greer and associates sells a 1.5 quart finishing resin kit for $24 plus $8 shipping. That's twice as much finishing epoxy as you get with the ZAP brand. It's thinner, cures to a clear hard finish and doesn't "blush" so it's suitable for finishing table tops if you want. It works great.

Check it out at www.jgreer.com