Welding Tips and Tricks

Greetings. I have an aluminum sailboat mast, 6160-T6, 3/16 wall, oval extrusion 10x7”. There are numerous 1/8” old rivet holes and a few 1/4” holes. There are 6, slots which are 1” x 3” round at both ends. In the corners, 4 old rivet holes which held a halyard exit plate now removed. My thinking is to insert an aluminum backing plate larger than the opening and use the old rivet holes and new rivets to keep it in place. I would fit a coupon equal to the thickness of the mast and TIG it to the mast wall. I’d leave the backing piece in place.
I want to keep the heat affected zone to a minimum. I have a Miller Dynasty 200 DX.
Any suggestions as to how best to weld this would be very appreciated.
Thank you. Mark

It wants as little filler as possible. 1/8" holes you have little choice but TIG. 4943 filler would be my choice. I'd counter bore the 1/4" holes to chamfer a bevel. Use a 1/4" rod ground to a small chamfer long enough to pass through to the other side. This will hold it in place, and draw some extra heat. Strip old welder cable 15 feet long and wrap to provide a heat sink.
Your bigger hole, make a patch out of thicker 6061, maybe 1/4" thick. Shape it, fit it flush. Use plenty of tacks.