How to weld seams in a a thin sheet rear fender

EVERY weld shrinks. In order to return it to its "flat" state, you must stretch EVERY weld back out by hammering on dolly and flattening it some. Generally, I would weld one spot every 3/4" and then go back and hammer each one using a dolly of the appropriateradius.

Then, grind each weld down to just proud of the base metal. After that, stack one more weld spot onto an existing one and work your way all the way around the seam. Then, hammer each one of those. Etc. Etc. After you think you are done, use a light to search for pinholes and weld those up. If you do it like this it will turn out pretty nice and you will get better as you go.

One last point. Use a high enough heat that all it takes is a quick burst to generate a full penetration weld. Don't just dab weld material on the seam. If you do it right, you will need to do a little sanding on the backside. I am assuming you are using MIG but the process is similar, regardless. Good luck.

I haven't done it myself, but I've watched two different body guys, and both did the same thing. Go back around, and around and around. First they leave enough room for 1 spot/tack and put a new tack centered like a quarter inch from the ones you already have. Alternating around the thing in 2 to 4 groupings before coming back to right next to the first one they did. Then keep going, skipping the gaps. When you fill in completely between the tacks you have, go around and fill in the small gaps with additional tacks to complete the seam. Time consuming, but it's the way to spread the heat around and not warp/shrink it (much).