I have been mig welding on my truck for a few years now and have experienced the same problems you are encountered with welding sheet metal. Here are the things that have helped me:
1. The prep and fitting of the repair piece is probably the most
time consuming part. But once it is fitted properly the
welding is a lot easier and neater.
2. In some cases using the next larger size of sheet metal in the
repair piece, when possible, makes the job easier.
3. Cut out all the bad metal before fitting the repair piece,
otherwise you will continue to "blow out" holes.
4. As mentioned before, tack don't try to lay a continuous bead
5. I saw an interesting technique on a TV show, where the welder
laid several tacks (on-off with the welder)one after another
and the resultant weld looked like a continuous bead.
Also, it is hard to say since I have never really timed it, bit
if you have a properly fitted the repair panel, a second and a
half seems like a long time for a sheet metal tack. In any case,
plenty of good advice in all the prior posts. And it does get
better with practice, as was mentioned many times.
