Stainless can be a real pain to work with, but it doesn't have to be. Unlike aluminum, with its soft characteristics, or steel, being so forgiving, stainless is tough—especially on tools. But if you can get past that, stainless can be a wonderful metal, and to try and make it a little easier to deal with, we went to Salinas Boys' Cole Foster for a few lessons on cutting, shaping, and even welding stainless.
Even if you're not up to the task of TIG welding stainless, per se, you can always do all the steps leading up to that point, then take your modified pieces to a proficient welder. But the welding itself isn't necessarily the most crucial part (though your alignment needs to be dead-on if you want the trim to stay straight and even). Things get really temperamental with the finish work, as stainless requires no chrome plating, so there's no copper filler to cover up any flaws. Polishing stainless is not as forgiving as aluminum or brass, but it's probably not quite as touchy as rubbing out a bumpy paintjob.
Let's start with the cutting. After you've determined the required amount of material that needs to be removed, mark your cuts as evenly as possible with masking tape. Make sure the cut marks are square to each other (we use an actual square to determine cut marks), as fit is very important. Either a bandsaw or die grinder with a slim cutoff wheel can be used to cut the material, but keep in mind that stainless likes to wear down blades quickly. Once both cuts are made, take the two pieces and align them on a flat plane to verify you're still on the straight and narrow (if not, carefully take whichever piece is a little off on a belt or vertical disc sander and take a little bit of material off at a time until you're "square").
Now, if you're going to weld the trim back together yourself, you'll want to set the pieces up (even in a little makeshift jig) so that they can't move around while you tack them together. We made up a couple different "contraptions" to hold down the trim without getting in the way, as you'll see in the photos. OK, being that stainless can vary in composition, you'll want to get both 308 and 309 stainless rod and test out before fully welding, as you don't want to see any of your mend marks when you're done. For final welding, putting a little weight on each piece aids in keeping them straight. The real trick is getting the weld on the backside as clean as the front, and to do that, we "purge" argon (at 1 pound of pressure) through some type of "tent" to direct flow. We weld the stainless as cool as possible; penetrate, but not burn through. You can also coat the backside with Type B welding flux to help obtain a much cleaner weld. (Untreated, the back of a stainless weld is messy and rough.) Your weld should be golden in color. After lifting off the pedal, let argon flow over (most TIGs have an "after flow" setting).
Grinding your newly welded piece is next. Again, you want to keep the piece weighted down so the grinder won't move it around and just end up causing you more work. Make sure your area is very well lit, and wear safety glasses! We start off with a die grinder (with a cutoff wheel) and take down only the bead, not any of the virgin surface (parent metal) on either side—just like working with a sheetmetal patch. For straightening purposes, do the same to the backside of the weld, as well.
Now it's time to file. With a medium-toothed flat file, make smooth strokes over the weld, but make sure the file remains flat (no edge grinds!). Don't get too carried away; keep checking the surface, as you would when doing bodywork. As a matter of fact, guidecoating the area with a spray bomb or even Dykem will let you know exactly where all the highs and lows are and, hopefully, not let you go too far. Before cutting into the stainless switch to sand paper (wrapped around a paint stick or even the file itself), starting with 120-grit. If you encounter any low spots, you can take an old Phillips screwdriver (or even a straight blade) and grinding tip to a dull point, put the part on a smooth surface like wood, set the point on low spot (which will be high on the back), and push the screwdriver carefully. If you need to tap it with a hammer, hit only the tool, never the stainless itself. Then, just take your 120-grit to see if your little mission was successful. Once you start seeing the weld blend nicely into the stainless, you're ready to move onto the DA sander. Using a DA in "locked" position (as opposed to oscillating) will ensure you take down the material even further. Again, start with 120-grit, and keep the sanding surface flat with the piece. Work back and forth, gradually working your way further out from the weld bead each time. Step up to 250-grit, then to 400, at which point all grind marks and scratches should be gone and you are looking at a shiny surface. Next, wet-sand with 600-grit prior to taking the piece to the buffing wheel with a stainless compound.
Hopefully, the steps described (along with the photos) will give you a better understanding of working with stainless trim.