I had exactly the same problem... on both rear wheel wells. I had them repainted, and two weeks later had to drive a few kms on a gravel road (ick!)... and the same thing happened again, not surprisingly. I never should have taken the car on that trip but I didn't know that the highway was going to be under construction. So I'd like to find a solution as well, for those times when you have to occasionally drive over a gravel road without going at a snail's pace.