ProStreetDriver

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This slideshow video outlines how to repair sheet metal. This demonstration was done on a lawn tractor mower deck, but the same principles can be applied to auto body work on vehicles. Most auto body panels do use a thinner gauge steel, so when you welding, more care will have to be taken to reduce the amount of heat distortion.

Removing the rusty soft/thin areas will allow for a solid area to weld the patch to. This also reduces the chance of having the welder burn through the existing steel.
After the hole has been cut, you are now able to make a patch for the damaged area. For this I purchased similar gauge/thickness sheet metal. To bent the steel, I used a couple concrete blocks for weight, a propane torch to make the steel more pliable, and a hammer for forming. To cut the patch to size I used an angle grinder and a file for any fine fitting adjustments.

The cuts in the one rounded patch allows for the piece to be bent into two different directions to match the contour of the mower deck.
Grind the paint off around the area where the patch will be welded to. This will ensure that welder is able to arc on the existing surface properly.

The tack welds hold the piece in place and allow me to align the patch to the existing surface. For the welding, I used a small flux core mig welder.

Running small/short (approx. 1" length) welding beads reduces the amount of distortion within the sheet metal which is caused by excessive heat. This will have to be adjusted accordingly depending on the gauge/thickness of steel.

For grinding down the welds I used the angle grinder with standard grinding disc for the higher areas and a flap disc to smoothen the surface.

If your applying this process to auto body panels, you can fill in the low areas with some type of filler to achieve a smooth finish. This was a lawn mower deck, so it was pointless for me to apply any filler.

For paint, I used low gloss black farm equipment painted which I brushed on.