I just use the heat gun with finishing epoxy and skip the liquid thinner. Depending on the epoxy you have, heat alone gets it pretty thin. You can wipe off the excess with a paper towel. Of course you need some finesse in the heating department, you don't wanna make it gas off.
If you must use liquid thinner, forget the drug store and buy a gallon of denatured alchohol from Tony's or use a bit of Everclear from the liquor cabinet. If Fat Al and his gang get the upper hand, you'll be able to siphon off a little from the fuel tank of your Smart Car.