I'm No welder by any stretch but can stick stuff together with out it falling apart.
And everyone has their own method for doing light guage metal.
The trick is to find what works best for you.
For me the voltage your using is not near high enough and is leaving welds that are sticking up.
On your last photos your getting closer in a couple spots to full penetration as you can see on the back side where the gap has been welded closed. That is what you want to strive for.
Give this vidieo a look at.
http://www.youtube.com/watch?v=JohBH...eature=related
I use the same method for body panels and all light gauge metals. High heat short weld time with a cool down in between each spot weld and after few spots let the area cool to prevent warpage.
My machine does not have a weld time setting but you can do that on your own and after some practice you can get the spot welds all identical.
I personally prefer this method as pentration is guarenteed and it leaves a low weld that is easy to dress. Perfect for body panels.
While your waiting for the most recent welded area to cool you can spot up another location if the piece is large enough. If you want to speed up the proesses you can help speed the cooling with blast from your air compressor.
That will aso help from over heating and warping the surrounding metal.
This is not the fastest method to join light gauge metal, but if you have to dress the weld, the time is recovered in dressing out the welded areas and can usally be done with just a flapper wheel on a grinder. The bonus is it does not take huge amounts of skill to master this technique.
Matthew