Slider, every hole has a character of its own so I will lay out some of my techniques. Firs, what is behind the hole will dictate some of your approach. If the hole is less than 1/4 inch, and I will not set anything on fire, I will use the mig to weld it up. This technique is a series of bips around the edges, followed with a longer hit in the middle to puddle the little bips (use caution here if metal is thin and fairly large panel, can cause warping). If the holes are larger, I start looking for filler which can be like you mentioned. I have used small magnets to help hold these flush with the surface until they are tacked in place, or will spot a piece of coat hanger to the patch and hold in place. Once tacked, I slowly bip arround the edge not letting the area get to hot. If there is a bunch of swiss cheese, then I cut this out and patch but if you can get behind the area, I sometimes just fiberglass the back side. There is a good product out called Allmetal that will also work great.
Trees