Jon,
Yes, it can be done.

First, safety issues; NO BRAKE CLEANER OR CHLORINATED HYDROCARBON SOLVENTS for cleaning, but clean the steel thoroughly. I use acetone and believe it to be safe; it is so volitile it is hardly an issue. Other cleaners particularly in automotive fields can generate the deadly gas phosgene, outlawed during WWI. I have heard of automatic dishwasher detergent and other soap type chemicals being used as well, but rust now becomes an issue due to the water.

You may want to take note of and consider new adhesive bonding techniques gaining popularity, which hold many advantages over welding in respect to controlling difficult issues including distortion control, rust resistance, etc. I cannot answer questions about them as I am only passingly familiar. They may or may not be appropriate. Crash safety is an important consideration.

Beyond the issue of burn through you will need to control warpage and distortion. Get plenty of practice before you start out on the actual job.

You can control burn-through by reducing the power, reducing the diameter of the electrode (which permits further reduartion of power), and/or by using a backing- a plate or bar of aluminium or copper in close contact with the sheetmetal to help absorb the heat... a good weld supply shop or bodyshop supply may have some products they can recommend.
I gain some control by modifying my technique to suit the application, as well.
You might gain some advantage by changing to a straight polarity (DC Electrode Negative) when using an AC or DCEN electrode. The negaitve pole tends to get the most of the heat from the arc. I will spare you the laws of physics governing this phenomenon, but there will be less penetration into the sheetmetal and more heat on the rod; it will weld differently (perhaps not better, but differently) and may be an advantage.

By ALL MEANS be sure your filter is the PROPER SHADE to allow you to see what is going on within the arc; you MUST be able to see what you are doing. The new gold reflective filters reflect more IR heat and keep it out of your eyes. It permits a bit more flexibility with lense shades and means less discomfort at lighter shades; your lense may need to be darker to see into the arc.

Distortion control is another issue. Technique (length of bead and where you run them and in what order) is important; it depends a lot on the shape you are welding. You may get some insight from a distortion problem on a trailer one of us was discussing in the "shop-talk" forum; try "goose"'s post I think. Unfortunately sheet, particularly thin sheet, can be more troublesome than hollow steel tube sections.

Be aware of additional issues which may arise if BMW used any special high strength alloy steels for their body panels.

Good luck, and I hope someone can lead you farther toward your goal. Note also that I have never heard anyone ridiculed or laughed at for asking a question here. We all are familiar with the learning process, so wellcome!

Regards,
d

This is one area of welding where I have a decent amount of experience with. In addition to restoring/rebuilding my race/show car, I also work part time (I'm a full time Mr. Mom) helping out at a good friends body shop. I mainly do the fabrication/restoration/welding work.

We use Mig welding on sheetmetal mostly, although we do use the panel adhesive in certain areas.

I personally would never want to try and weld sheetmetal with a stick welder. I typically have to pulse the Mig arc on/off to control the puddle which would be very difficult to accomoplish with a stick (not impossible, but quite difficult). We try to use plug welds on complete panel replacements to duplicate how the factory assembled the sheetmetal. This makes for a strong, yet attractive job.

I just finished a bunch of work on a '70 Mercedes Benz convertable. The German cars are definately assembled differently than the American cars. If you give us an idea of what specific parts your going to weld on, one of use should be able to give you an idea of how it can be done the best. Typically, I look for the factory spot welds/seams and drill out/cut panels out at these locations and use these same locations to weld the new panel back in.