RASelkirk, You should be more specific. When you say negative polarity is that the ground or torch polarity?
If all else fails try a sieries of tacks connecting one to antother to form a bead. It's much easier to not burn through and is just as strong as a bead when mending thinner metals like body panels. Also tune in your welder by sound, it should sound like bacon sizzling(steady and smooth) when it's tuned in. Good luck...
Welding ATO You did not state what kind of helmet you are using. I have been welding since circa 1954 and the best helmet I have ever used is the photo sense helmet. They are great because you can see what and where you are welding prior to the flash. If you are using just an old shade glass helmet you are at a disadvantage. Texasjack