No matter what welding process you use, it’s important to finish the job by cleaning up and putting some kind of protection on our welds.
Heating up the metal removes the old surface layer and any protective coating, that might have been applied to the outside of the metal, such as paint. The exposed metal then oxidizes and becomes rusty. And, in a humid climate, it doesn’t take long for the rust to start eating away at the weld.
I like to power brush the weld with a grinder. You can buy a cupped wire brush that attaches to the small hand grinders made by Milwaukee and Porter Cable. Make sure the brush you buy has a speed rating above what your grinder will put out.
Just run the brush over the weld to remove any slag that formed, splatter, or other surface imperfections. You could also use an abrasive grinding wheel to actually smooth out the weld, if you wanted.
Once the weld is clean, use a brush or sprayer to cover the area with zinc-chromate primer, to inhibit corrosion. If you want, you can sand the primer to remove bubbles and make the surface smooth.
Finish up by applying a couple of coats of paint. Enamel gives a hard, shiny appearance. You can also get enamel in matte or flat finish.
That’s about all you need to keep your handiwork from rusting away into oblivion. Don’t forget to do the back side too, at least with primer.
You could also take the part to a plating shop and get it either plated or anodized. We usually think of chrome when we think of plating. Anodizing comes in all kinds of colors and gives a very high tech look to metal parts. Chrome obviously protects steel from corrosion, although it does need waxing for a long life. Anodizing also offers protection from corrosion. Talk to your plating company for details.