How do you properly grind welds in sheet metal? What type of discs or tools to use?

[BigFlo]

I would say it’s normal to have to smooth over every weld. Everyone that’s welded 50 year old sheet metal that has been cleaned of rust, cleaned of paint and has tried to weld it to new clean metal knows that it’s a battle - and a constant feedback loop. Weld settings may need to be changed as you go along the same piece depending on changes in thickness. It’s not easy, but as you mentioned KD, getting the best weld is the best start. But welding is forgivable - if you blow a hole through the metal you can build it back up and fill it in.
After that I would say find the tools you are most comfortable using and maneuvering. For removing lots of material quickly, I used an angle grinder with a flap disc and then an angled die grinder with 2 or 3” discs for the rest.

 

[BigFlo]

Butt joints - yes! Agreed that lap joint invites corrosion. I tried those panel clamps you show in your picture, but I found the gap they create is too large for sheet metal. I could not get a good weld with those - maybe operator error cause I’m no expert welder. I used those really strong magnets to hold the joints together on my lower quarter patches - they worked really well. I was able to have near zero gap in the joint which helped my welding. I also used copper backing plates where I had access the the backside of the panels to help draw the heat away from the weld.

 

[Llamaback]

I really like using the carbide burrs from Kodiak in my die grinder Carbide Burr Sets | Double Cut Carbide Burrs | Kodiak Cutting Tools. They cut the weld like butter and I have a lot of control with them. I'll get my welds real close to the sheet metal surface and then switch to a 2'' disc with either 40 grit or 80 grit. In my opinion the guys that show you perfect appearing patch panels where they make it perfectly smooth are going to have some thin spots in the metal. I do my butt welding planning on having my weld slightly low so I can seal the weld with epoxy and then put a skim coat of duraglass on top of it.

 

[Kern Dog]

There was a 68 Charger that was used in the end of the 7th Fast and Furious movie. The car was named MAXIMUS.
It is an unpainted steel body where they dressed every bit of metal work with tools but no filler, primer or paint.
I’ll never be that skilled but it doesn’t hurt to try to improve my skills.

 

[Kern Dog]

Here is that car.

It obviously has a clearcoat over the steel.

The rear of the car was widened. I don't recall how much but look at the shoulders of the quarter panels near the C pillars. Charger owners will notice that it is wider there on this car.

One thing that I do like about these custom Charger builds is the attention to the gaps around the bumpers. Note how tight the one above fits compared to a stock bumper.

 

[70chall440]

36 - 40 grit flap wheel until the weld is almost flat. Move the angle grinder around so as not to induce a large amount of heat into the panel.

Then come back with the purple 3M rolox discs to blend in the weld to the base metal.

Lastly go over the weld with a DA with 80 grit to remove the bigger scratches and fully blend the weld.

This produces superb results if done correctly. That said you also have to accept that not every weld is going to disappear completely and if on an outside panel may need some filler.

Another “tip” is that if possible hammer and dolly the weld as you get it near flush which will also help to even it out. Don’t need to go caveman on hitting it, just light hits to correct any wavering done but the welding and grinding process.

 

[69RunnerMike]

I've tried grinding the welds but have had far better and quicker results using resin fiber discs. I knock the welds down with 36 grit and then follow up with 120 grit. If I want it to look real pretty, I follow up with 220 then 400 grit. I use the discs from Benchmark Abrasives. They are thick and stiff and seem to last forever. The only problem is heat build up. I'm going to take a look at the MP&C threads.

 

[BigFlo]

If you are going to apply any filler over the weld I wouldn’t recommend sanding anything finer than 120 grit. The filler needs something to hold onto.

 

[Llamaback]

36 - 40 grit flap wheel until the weld is almost flat. Move the angle grinder around so as not to induce a large amount of heat into the panel.

Then come back with the purple 3M rolox discs to blend in the weld to the base metal.

Lastly go over the weld with a DA with 80 grit to remove the bigger scratches and fully blend the weld.

This produces superb results if done correctly. That said you also have to accept that not every weld is going to disappear completely and if on an outside panel may need some filler.

Another “tip” is that if possible hammer and dolly the weld as you get it near flush which will also help to even it out. Don’t need to go caveman on hitting it, just light hits to correct any wavering done but the welding and grinding process. This can also provide the added advantage of slightly shrinking the metal by making it thicker.

I don't see how hammer and dolly work, especially hammer on dolly could possibly shrink an area around a butt weld. Sure bumping it around could help straighten it out but would likely cause a little bit of stretching instead.

 

[70chall440]

I don't see how hammer and dolly work, especially hammer on dolly could possibly shrink an area around a butt weld. Sure bumping it around could help straighten it out but would likely cause a little bit of stretching instead.

Yeah sorry I meant stretching...