[mongocanfly]

I kinda skipped thru the replys , but I think most has been covered...
using copper for a backer will greatly improve the weld quality
if your using flux core, if possible,don't, get gas shielding and it will greatly improve...not saying you cant flux core , but gas is better
I did my share of thin metal welding and used both .023 and .030 wire..the.023 did better on the thinner metal if I used the copper backer..
when grinding , don't grind your existing metal, grind only the weld you added.. I found the best way was what mp&c recommended ..use a cutoff disc in a air grinder..hold the disc 90deg to your weld seam and it will allow you to stay on top of the weld and off the fender metal...hammer and dolly your welds..when you weld thin metal it tends to draw up..hammer and dolly to stretch the metal back out..