Thinning P38 with acetone will more or less work, but this also slows down hardening, and it doesn't do much for the properties of the stuff when cured, you may find it will crack over time. Because acetone evaporates so fast, you'll also find any remaining filler is practically solid next time you open the tin. A much better plan is to remember to keep the lid on tight to stop it drying out in the first place, and if you want to do the best possible job, throw away hardened P38 and treat yourself to a fresh tin, you'll probably be glad you did in the long run. If you're only going to use small amounts, it keeps better in a tube. What causes drying out in the first place is evaporation of one component of the resin base, something called styrene monomer. Thinning the stuff by stirring in more styrene is a laborious process, but it's the only way to return the material to it's original state. You can also thin hardened filler by stirring in standard polyester resin, but this will make the stuff so hard to sand, that you'll probably wish you hadn't bothered.