...Just bought a regular leather jacket at a thrift store for $6. Slick satin like lining inside and a black suede outside.
Will this work? Should I remove the lining? Any other alterations to make?
First, as asked above, is it real leather? The label should indicate what it is. Synthetic suede (like amara) is machine washable. I really like the stuff on my mechanics gloves, but it isn't ok for welding.
Next, the lining has got to go. Thermoplastic fibers (polyester, nylon, etc.) will not only burn, but can also melt and then fuse to your skin. BAD stuff. Cotton will burn, but that's only a small fraction of the danger from synthetic fibers. So, a cotton shirt under something FR is fine, but no polyester shirts (or underarmor, etc). Got it?
With the lining, the cuffs and any fabric collar has to go. Pockets are a problem, and should really be stitched shut. The zipper is probably assembled on a synthetic fabric tape. Metal snaps are ok. I'd figure an hour with a seam ripper (while watching a TV show with the SO) would have it down to just the leather.
Finally, real welding leathers (if you really want leather to weld in) will be stitched with kevlar thread. When one berry hits the thread on your thrift store jacket's seam, it's going to open up, and you'll be unpleasantly surprised.
I've got a Tillman leather welding jacket. It's nice in the winter, but that's about it.