Smooth butt joint

Looking for suggestions. Whenever I have a welded butt joint I find it hard to get it smooth… usually end up with a divot from the flap disc on the angle grinder.
I’m sure my technique could use improvement but I see some people using sanders.
I bought a DeWalt dual action pneumatic sander and it was disappointing. It would sand but didn’t have much torque and would loose speed easily.
Is there a better quality DA sander that would help me flatten out a weld seam, or is there another tool that would work better?


you probably have some warpage and this will make it hard to get flat. show a picture of the joint your trying to grind flat. Maybe I can give some pointers as a lot of what I do requires welds being ground flat.

Here are 2 photos of the same joint. I think I am gouging with the angle grinder and that’s where I thought a broader sanding pad would serve me better.
It really stands out with a gloss paint, and you can feel ripples when you’re on your fingers along it

Are these joints that have to have the weld ground or just for looks?

Maybe change process to get a flatter weld like tig.

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Yeah, that’s definitely material being removed by grinding. What media are you using in the grinder? Hard wheel, flap disc or something else?

Grinders can be too aggressive if you’re trying to preserve the original profile of the metal. You can try finer grit material or switch to a different tool like a die grinder where you have more control.

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A flap disc on a grinder is the best way to grind down welds but it requires some practice.

Usually, I will hold the grinder with a 40G disc perpendicular to the weld. Feather the weld down close to the surface while holding the grinder/ disc as flat to the surface as possible. You will start to see a “halo” around the weld area. At that time, switch to your DA with 40G pad and start to sand in a bigger area, not just where the weld is. Switch to 80G to get smoother finish. (Think mudding drywall seams)

Remember, that welding will always “sink” the area around the weld itself. This is from the HAZ causing the grain structure to shrink and/ or elongate. So, 100% flat is really hard to achieve.

Also, it looks like you are using hot roll. If you are grinding the scale off before you are welding, there is a chance you gouging the steel. Even grinding through scale, the grinder puts scratches in the metal that could affect the HAZ causing more shrinkage.

You could also use a 90 degree die grinder with Rolocs. many sizes and easier to handle.

And if all else fails, you can always Bondo and sand. :grin:

Happy Grinding


You are correct that is from holding the grinder on a angle to the surface.
here is a couple of video’s of different techniques and tips

This picture shows how a flap disk looks in use and as you can see it does not stay flat over the weld bead. That is why I start with a fiber disk or a disk with a backing pad as they will stay flat.

I use a fiber disk to knock the weld bead down, and soon as it starts touching outside the bead I stop and switch to a flap disk ( I use type 27)
I will use a 40-60 grit flap disk and very lightly start on the weld bead and slowly move out a couple of inch’s and each pass go just a little further. You may need to feather it back 6-8"
Then I might move to 60-80 grit and make a few very light passes. Make sure you keep the disk almost flat to the surface.
Pay attention to pattern the grinder is leaving behind and make sure your not grinding to much in one spot.
Then I might follow up with a DA and 80-120 grit
If your weld profile seems pretty high you could always grind a bevel (depending on thickness of material) to help bring it down some.
don’t be in a hurry and if your painting you can always add filler, but that requires correct sanding too
Old worn disks will make it harder to get good results also


Thanks for the tips I will check those videos out as well. I can imagine my technique could use improvement.

What is a good brand of a DA? I tried a pneumatic dewalt and was dissapointed.

I just use a cheap porter cable electric. If you want one that grinds you need one that you can lock the pad, if they still make them. I had a National Detroit that you could lock, but that was 35 years ago

I start with a flap disk and finish with a pneumatic DA also. I have this Aircat, works great going on 3 years now.


I use a pneumatic sander as well.