Mine does that every time I fill up or trailer it with a lot of fuel. Body shop paint supply stores sell a real fine buffing compound you can rub on by hand with a damp rag. It takes the stain right off of mine and does not scrach it a bit. If the stain has been there for a long time you might have to use a power buffer. I keep it good and waxed around the filler to help protect against fuel damage to the gell coat.