I dont think anybody pointed this out yet but if you are trying to weld aluminium make sure your mig welder has an A/c setting as you need that type of current to weld. You want to preheat the area first clean it with a stainless steel brush and if it apiece that has thick and thin parts of metal fairly close to each other you may want to post heat aswell to avoid cracking. Get your self some scrap aluminium to practice on as you will see that when you weld aluminium you have to travel at a greater speed than you do when weldind carbon steel or stainless. Hope this helps mate and good luck.