I would continue to soak and lightly scrub. And by that I mean soak it, not put a wet paper towel on it. You can go out and buy a gallon of vinegar for $3, fill the tank with a vinegar/water solution, and let it sit for a day or two. Try scrubbing it while its full. I would also add an airline without the airstone to keep the water moving a little. I say that because I have had airstones dissolve on me before and you don't want that grit in the water while you are scrubbing, but you want the water to be moving somewhat.

Acrylic polish would work as well, but it may leave it a little hazy.