In preparation for a local show, I took both my mustangs down to gas them up the day before, because it was a long drive. It was a hot day and I had put them both in the garage because I had cleaned them for the show to follow in the morning. The gas in the tank apparently expanded and was leaking out the back gas cap. Not all that unusual, I have seen that before. This time , however the gas comopletely stripped the paint below the cap on the back valence. I have never seen this before. Has anyone else seen this ?  Now I have two mustangs that I have to repair the back tailight panel on.  I cant believe how fast it worked. I have used paint stripper that didnt work as well. I have seen discoloration from gas before but never have I seen it strip paint like this. It was 10% ethanol, but that should not strip paint.  

I've never seen it strip it either, only discolor it. Obviously there must have been an interaction between the gas and the paint at a molecular level. Sorry to hear about the damage. I would be pretty upset also.
JLG  

Here's what happened. Your car must be painted with a clear coat or you have some lacquer primer used in this area. Gas getting on top the clear finish will in most cases do no damage, or at a minimum discolor the clear. However, gas that can get in under the clear finish, such as around the filler area where it would have been masked off for paint can cause the peeling issue. If your car has a base clear coat finish, the base color is thermo plastic, or what I like to say, it’s a sponge and will soak up solvents (gas). The problem is, the gas will not evaporate the same why in soaked in. This causes the paint to blister. Hope this helps. The best future fix is to have the clear extend beyond the color on an edge. A sixteenth of an inch is all that is needed.