PLASMA CUTTING AND MIG REALLY DON'T DO WELL OVER PAINT AND RUST, NOT TO MENTION THE FUMES FROM BURNING PAINT ARE HAZARDOUS AT THE MOST AND BOTHERSOME AT THE LEAST. YOU'RE BETTER OFF IF YOU GET AN ANGLE GRINDER WITH A WIRE BRUSH ON IT TO REMOVE THE PAINT AND RUST FIRST. IT DOESNT CUT INTO OR DAMAGE THE THIN STEEL THAT YOU'RE WORKING WITH. HOPE THAT HELPS. HAPPY ART WELDING!
-BUFFALO

Artgirl,
Aside from the potential health risks, paint and other contaminants (rust, oil, mill scale, etc.) do not react well with weld metal. They can allow the weld to pick up hydrogen and other materials which can cause inclusions, porosity and/or cracking. So I would recommend removing the paint, oil or other contaminants for at least 2" (50 mm) on either side of the joint to be welded.
-Sean

Stay upwind if you can! Who knows what is in various coatings. As a blacksmith I've gotten into some lousy fumes even when I thought the metal looked reasonably clean (be careful with pipe/tubing too, I've had some heavy smoke come out of old pipe/tubing).

There is a person named Ron Young who specializes in various coatings and is an expert with patinas, primers and protective coatings. His web site is www.ronyoungpatina.com or www.sculptnouveau.com I have been to some of his presentations and used some of his techniques/products.

If you want more info let me know at accentmetal_art@yahoo.com
Ken

Artgirl,
sorry for content and grammar; I am having connection problems

consider oxyacetylene cutting it burns off all contaminatnts, paints, oils, plating films, etc; consider all dusts and fumes toxic; same as artists paint has cadmium chromium, etc are toxic so do metal finishes so presume the worst for safety's sake

contact DuPont or Sherwin-Williams or other paint manufacturer- rrequest techincal information on their metal finishes; do not rely on advice from counter sales staff get the info in printed form from the source... they are designed as a syatem to be compatable with their recxommended cleaning chemicals, primers, and finishes as the chemistry varies; I mean REALLY varies. There are EPA regulations at federal, state, and local levels small users may be exempt from... find out about regulations

These finishes present hazards to safety, health and the environment and use of them requires proficient familiarity with them as well as a means of manageing that risk.

As previously stated the chemistry varies... and offers various advantages or disadvantages such as UV stability, flexibility, film quality, gloss, build (thickness) characteristics, etc.
This is one reason you wilol want the technical descriptions and performance data of the finishes offered. I suspect a polyurethane will be your final choice due to convenience over the more durable epoxies, but my expertise is rapidly becoming dated... traditional nitrocellulose laquer will turn to crap rapidly in constant exposure to extremes of sun and elements when compared to newer chemistry, and, frankly in my opinion water-borne finishes are useless without close and impractical controll over humidity, etc. With the proper chemical additives I can laquer in any temperature at any humidity with good to outstanding results, and handle the product within minutes. Water-borne cannot compete. Powder coats may, I am not equipped for nor am I familiar with them, but also investigate how the finish fails (does it darken, loose gloss or transparency, or flake off in big sheets)

Email for specific questions /details is OK
Regards
d