Photo:
You are correct. Welding will discolor the angle where it is welded. You try to minimize the discoloring and scaling by using an aluminum or copper backup block on the angle as shown in the instruction. Depending on how bad the scale is, use progressively finer wet-or-dry paper to at least 600, finer if you have it, and then polish to a shine with a liquid metal polish. There will likely be some distortion at each of the welds that you can't polish out.
Mar-Ktech
