[MPWR]

It's very much like KevHw said. You're applying the paint too thick and heavy. Propellant dissolved in the paint will bubble out like that if the paint is allowed to gather any thickness. I'm guessing there is some thick buildup on the fender? This is one of the many reasons that paint must be applied in slow, light mist coats.

Once bubbles appear like that, there really isn't a good way to fix them. You could try to cover them with a decal or the like.