I pulled out my top wing skins and one of W-00002 has this spot beneath the vinyl. I rubbed it off with some 800 grit. It seemed like it was just some dirt. There isn't an indentation in the spot after removing. Build on?
Thanks for the heads up on the grit, it's 3M 213Q. I should have been less invasive, think it would of wiped off with water actually.I don't think it is a problem because it looks like it is on the surface only and it should not be sanded out by more than 10% of the thickness of the sheet (see AC43.13-1B).
If using 800 grit then ensure it is not silicon carbide "wet and dry" because the carbide grit can embed and cause corrosion on aluminum. Scotch brite (aluminum oxide) is a safer bet.