It's unlikely that a fuel tank will be made of an identical fibre/resin/coating mix as the rest of the aeroplane - superficially it may be similar but in all likelihood there is a different makeup to account for the continuous fuel exposure.
Additionally, two things that don't do "plastics" much good are UV exposure and chemical exposure. On the wing it gets the former, in the tank it gets the latter. Provide both and there's at-least some chance of things going on that are untested, and certainly not allowed for by the designer.
And, all-else being equal, it'll probably stain and make the aeroplane look ugly.
G