acetone but it evaporates quickly ,styrene monomer is better but hard to find and expensive,alchol works with epoxies but not polyesterway back when I was building ducted fans we used k&b OR hobbypoxy thinner 50/50.one trick stafford used and recommended on his b-24 was to apply the resin full strength then use a roll of toilet paper and roll it over the resin to pick up the excess,just keep peeling off the saturated tissue till nothing is picked up.