I've used ethanol which is almost pure alcohol, but the supplier here in Japan, a paint outlet, only sells an ethanol/acetone mix, 75/25%.

Acetone is used to clean up the brushes etc., so you should be able to use straight acetone to thin epoxy if you need to.

Don't use acetone around foam as it melts the foam. I built a foam cored flying boat, and 25% acetone is OK, but I wouldn't go any higher.