Grinding Down Weld Beads.

[satexasuser]

I"m having to do a 5 foot butt weld section in the inside of my crew cab. I wan't to really grind down the weld beads smooth. What kind of grinding discs(20,40,100 grit) would you use to get those weld beads smooth? Also once those are done what is the next step for preping those welds for primer? or is Primer the next step?
picture of weld beads I want to grind down smooth and prep for painting

[Matt Harwood]

Grinding weld beads well is almost as difficult as doing the butt weld itself. I've spent the last several months rebuilding the trunk of my '41 Buick, and if you look at my restoration journal, you'll see how trying it can be. I've come to the conclusion that it is best to grind too little, even if it means the bead is still slightly visible. Grinding too much makes the metal too thin and you'll have to start all over.

That said, I usually use the edge of my cut-off wheel on my die grinder to dress the welds, then finish with an 80-grit disc. Move the cut-off wheel perpendicular to the bead and gently grind it down. Sometimes I'll stack two wheels on the grinder to give it a slightly wider "footprint" and slow down the cutting action. Once you have the weld mostly ground down, I use a hammer and dolly to smash it flatter, then finish the area gently with the 80-grit discs. This is where you'll do the most damage if you're not careful, so stop early, even if you think it isn't perfect. It's better than chasing a bunch of thin spots where you burned through the metal, which is what I did for months trying to achieve perfection. A little lead filler is fine for these spots, so don't try to make it all perfect with welding and grinding.

Check out my page (in signature) to see more of how I deal with this situation. Hope this helps.

[CARS]

I agree with Matt. Use a cut-off tool for the majority of the work and finish it off with your disc. I have doubled-up my 3" cut-off discs in the past. The wider you have, the less it gouges. I guess using 80 grit discs to grind the remainder of the weld is a good idea. I personally use 50 grit but I grind almost on a daily basis. For the hobbiest 80 grit is a safer way to go. Like Matt said you can really ruin a panel if you get the metal too thin.
Norton makes a really neet disc and backing plate that has holes in them so you can see your progress while it spins! It's spendy but cool.

[dough]

I am new to this. As a matter of fact if you scroll down this page you will see me listed as a "novice" and to nclude me in your emails.

I am truly a novice, at 60 years old and never had a Hot Rod. We also have custody of two of our grandchildren ,7 & 10 who take up most of our time.

I read just about anything I find about cars and watch a lot of car shows on TV. I am attempting to build my first Street Rod. A 34 Street Beast 3 window coupe. I bought what I could afford.

I have been grinding away on the weld beads, especially in the engine compartment of the frame. Figure that is what most people will see. The other night I was watching the Boyd Coddington show, not that I could afford any of his stuff, and saw his body and paint shop man use body filler around the wells to smooth them out. He applied a little to his finger and ran a bead of filler along the lenght of the weld Once it dried he sanded it down and you would never know there was a weld bead there. He said the trick was not to use to much filler as that would be more work than trying to grind the bead.

What do you think? Good idea or keep gringing? Or both? It seems to me the best of both worlds would be to grind down the biggest part of the bead, then fill it in wilh filler to smooth it out. That way you would not have to worry about grinding too much off.

Doug Harmon

[satexasuser]

For me, I'm going to grind down as much as possible without making the weld bead so weak it's not useful. But I want that weld bead down smooth(flush). Also I'm going to be able to weld on the backside of the butt weld so that will allow me to get pretty close to flush. I'm not sure about the other item you mentioned, but I'm going to keep on grinding....

[Matt Harwood]

For me, I'm going to grind down as much as possible without making the weld bead so weak it's not useful. But I want that weld bead down smooth(flush). Also I'm going to be able to weld on the backside of the butt weld so that will allow me to get pretty close to flush. I'm not sure about the other item you mentioned, but I'm going to keep on grinding....

If you keep going, remember that the sheet metal around the weld will get thinner, too. When I had problems grinding my welds, it was ALWAYS with the surrounding area, not the weld itself. The weld is far harder than the sheet metal, and it's hard to tell how much of the sheet metal has been worn away by grinding, while it's easy to see that you still have a weld bead.

The sheet metal around the weld will eventually be paper thin, and if it starts to discolor or turn blue, you've already gone too far. It's better, as someone mentioned above, to stop grinding early and use some filler of some kind to hide the weld than to grind too much and have to cut out a bunch of formerly good metal and weld in another patch.

Believe me, I've done it more than a few times on my Buick, always thinking that I could just...make...it...perfect with more grinding. WRONG! If you don't listen to any advice ever again, at least listen to this--too little is better than too much!

[satexasuser]

Matt, Thanks a bunch. If this rain will stop I will finish up the last 3 inches of the filler butt weld line and then start to gentlly grind. But one question, once you got it all ground down with bare metal exposed, what do you do next, after cleaing up the butt weld line? clean up then put on some spray on primer? I can't seem to get any answer on this one. Since I'm going to weld on the backside of this butt weld line, I'm going to have to cover this up also. Any suggestions?

[satexasuser]

ok matt I took my angle grinder and put in a wider grinder blade an got busy. After I got the butt welds beads down close to flush I took some 100 grit on my angle grinder and took it down to flush. What I noticed was: In a few areas had some hair line cracks and a few small holes. I found this new setting on my mig which mad wdling as smooth as laying on butter with great penetration. So I went afte those few areas. and Then got after the underside of the cab. Man tht welding lying down trying to keep the gun going was fun, not to metnion the dogs trying to steal a little smooch every now and then. But it's looking good.

[Ultra Al]

Just a comment. This has been a very informative thread for me, a lot of good info here. THanks guys. BTW Satex, I can just picture you laying there with your MIG with the dogs licking your face, LOL gotta love them dogs. Al

[satexasuser]

alright, I took a 60 grit flap disc to some of the weld lines and man that was some sweet stuff. It really allowed me to get those weld lines down and the go back and touch it up again. What I did then was take some 120 grit wheel and kind of featherd them in starting to look really nice. Now I udnerstand the differnece between flux-cored and gas shield on the migs. With the gas very little grind down time.