As said above cut out rust and weld in new metal . BUT there is an alternative method . If you can get to the back side of the car panel? Cut out the area to be repaired. Clean the back side of the opening, prepair your patch larger than the hole. GLUE in the patch, secure with selftapping screws, and let dry. After dry remove screws fill screw holes with bondo while covering the seam that is there. You can flange the new patch or the body or just fill it all over with a skin coat. You would not beleave how much of this is done today in body shops and in the manufacture of new cars. Welding can buckel a large panel and its hell getting it back. Its a two part adhesive made by 3M #8116 it comes in a tube type thing and mixes as it goes through the gun which is part # N.08117. I glued to pices of sheet metal together with this stuff and it does not come apart