I'm seaming the fenders on my '51 Ford. I have the heat on my mig welder (Lincoln 175P) turned all the way down, but it is still frying through the fender and trunk panels (on each side of the seam which I am trying to seal). I've joined the two panels pretty good, but I am having trouble filling the "fry" holes. Am I doing something wrong here? Should I try to back-fill the holes by adding a sheet metal strip on the inside, or just smooth it out with body filler and call it a day?