Finishing a weld

The repair in the photo is a repair to the top of a wing (fender) mounting panel. It's made from 1.0mm mild steel and I've made the repair section from 1.2mm sheet.

The real trick in a nicely finished weld is prep work. This is a butt weld (edge to edge) and I took some time making sure the top edges were level while tacking up.

I've always used angle grinding discs to finish welds, but the ordinary grinding discs are very coarse and difficult to control, so I tended to grind almost back to the parent metal then use a little body filler.

This is a (slightly worn - they come with straight edges) 40 grit flap disc. These are the way forward for finishing welds. If you don't have one then go out and get one.

Flap discs are flexible, and will tend to follow the contours of whatever you are grinding. Not much use if you want to get rid of those contours. Trick is angling the disc (maybe 20 degrees for this job) and using light pressure.

Even with light pressure they are still quicker than a grinding disc.

The flexibility makes them directionally sensitive too. On a curve they seem to cut more from before the weld (as the disc spins). Here I've ground the weld just about level with the thicker repair section.

Next step was to grind the weld into a curve until I met the next panel.

Close comparison of this photo and the ones above show the weld has been finished, but the metal on either side has hardly been touched. There are still the couple of scribing marks to the right of the weld which are more visible if you enlarge the photos by double clicking them.

The advantage of the flap disc over the grinding disc is amazing.

A tiny bit more finishing and the weld is invisible, yet the panel thickness hasn't been compromised. I'll be able to paint that without using any body filler.

Got to be careful though - if everything isn't level to start with one sheet will be thinned getting it to this finish. In that case it would be better to go so far then finish with body filler.