Problems removing a rim that surrounded the removed decal.

I’m changing the graphics on my hull. I’ve got the old decals off (heat gun to peel off most of it, then wiped off residual adhesive with gasoline).

What I have is a faint white trace outline tracing the silhouette of the removed decal. I supposed it was a faint line of adhesive residue. So I attacked the trace outline with more gasoline. Then I tried Windex, then rubbing alcohol. Then I guessed that maybe it wasn’t adhesive residue, but lime residue. So I attacked it with Babe’s boat bright (which works great on lime spots for our hard water). But nothing seems to be working very well.

So I got down close and studied this trace line real close (it’s an interlacing network of oval and french curves according the silhouette of the removed tribal decal). I scratched my fingernail across it both ways. What I’ve figures out is this.

The portions of hull that were COVERED by the decal are RELATIVELY RECESSED while the portions of hull that were EXPOSED are RELATIVELY RAISED. The elevation change is no more than half the thickness of a piece of paper. The trace outline is the escarpment ... so to speak ... between these too elevations.

Any ideas how this happened? Are the raised areas oxidation? Which is not my first guess because, the hull is the same color in both areas.

More important, how can this be remedied?