Although similar all epoxies differ in their contents and react a little differently, so the best advice is always to check with the manufacturer instead of relying on second hand diy-experience.
I know from a range of epoxies that I have been using that acetone will severely harm the cure, rendering the expensive product useless. In some cases acetone will even inhibit cure completely.
There are epoxies on the market that are as thin as water – I use S3 clear coat for example – and it'll be a lot less harmful than mixing acetone into epoxy. Acetone really is bad stuff (btw acetone will also go through your nitrile gloves without effort and once it has passed, the epoxy will follow easily…)