Agree with Coprhead. Except it did just occur to me that maybe this is something we've been showing wrong. If the wing itself is already dirty and grimey, then the spilled fuel would actually sort of clean it (ever use gas or alcohol as a solvent?) and then evaporate.
Also though, the constant spilling onto a painted surface would have to wear then paint pretty badly, so the area of the fuel spills should be a faded color if it's a painted surface.
Thanks for bringing this up