I'm assuming that you didn't tape off the hole and the gas is getting to the fresh paint and eating it up. There are a few different types of filler necks. Mine is a flatside harley tank that is the stock flush neck. On that style it has an outer ring that gets paint and the inner ring that doesn't. I tape it off with 2" tape and go around the contour with a razor blade. Never had a problem. I just had a repaint where the previous painter shot the neck (this tank is an older HD tank with the tall filler neck) and didn't tape off anything and it started peeling. I took a wire brush to the very top of it and taped off the same way. Sometimes it's not even the painters fault, but bad caps that leak.. The inside rubber piece in the cap should not come in contact with any paint, just metal -- If you use that as a rule of thumb you'll be ok!!
Good Luck! Bones!~
