How melted is melted?
Acetone will work like you said, but you dont want to dunk, but you want to apply with a brush. hmmm. a quick google tells me that acetone is pretty much top in its form. So a few suggestions would be.
Some sort of gel paint thinner, the gel will be reasonably strong and good at staying in the applicated area; rather than spreading across the model in anyway.
Heat? good old heat in concentrated areas?