If you use alcohol get the 91% isopropyl, solvent or stove fuel alcohol. Rubbing alcohol is 50% water. I thin epoxy with acetone. Not sure what a polyester resin would thin best with of those two choices. I know they use styrene for a thinner as one option. Test either acetone or alcohol well before relying on it.

Mix it "pure" first and let it set for a bit and then add the alcohol or acetone.