Thanks. The reason I wondered is I just got a slabbed coin that is quite attractively toned, but it seems to me there's a slight haze on parts of the obverse—the mint luster still shines through but it seems duller than it ought to be given the overall condition. I've seen similar coins brighten up after an acetone bath, and if it weren't slabbed I'd be giving it that treatment.
Anyway, since it is slabbed, I guess I have two options—just leave it be, or consider sending it to NGC and asking them to conserve and reholder it. Depending on the price it might be worth it.