I have had this happen in home improvement areas but not on a plane but I think this could help. It seems as if like you said, contamination has penetrated deaper than you are sanding down. I have had to sand down a little further than i care to and go over the area with balsa or wood filler very lightly then repaint/clear coat. This could also be from your treating only the affected spot. This could lead to a diffrent finish their than the rest of the area. You could sand down the whole surface, sand/fill the specific spot, resand whole surface then paint/clear coat. Or cover that spot with a nice decal and move on. Another thought could to be clear coat, sand lightly and apply a second coat.