Simple question, but I don't know the answer. What should I do about the back side of a mig butt weld? Should I grind it down as well? I tried hammering the joint, but its nice and lumpy from the welds on the backside? That said, at what point have I overworked the metal?

I'm not an expert welder by any means ... just completed a night course at the local ag college.

Butt welds are tricky alright.
You want enough heat to acheive decent penetration and not just have your weld laying on top looking like rope. Unfortunately, it's quite likely that some of the deposited metal will leak right through if the seams aren't perfect.

If this is 22 ga auto body sheet metal that we're talking about ... I think that I would be more concerned about having a good solid weld on the top and grinding just the part that you can see nice and level.

I was taught to grind down both sides almost all the way smooth,leaving just a small amount of weld, then hammer weld on dolly to remove the shrink and tension caused by the weld. then grind both sides smooth.

What I do is grind it down with a flap disc. Here's a page describing the process and outcome: Finishing Welds Link Hope it helps, I came across this years ago and never looked back.

Yeah, I was reading that. I can't get to the back side with a flap disk though. Its a late model and quite thin steel.

Can you get at the back side with a coarse half round file? I dont but weld newer cars like yours,I flange the joint for extra strength ,punch holes and rosette weld for less warpage or you can use a backing strip of copper when butt welding (the weld wont stick to copper,it also absorbs the heat) or weld a piece of sheetmetal for a backing strip 3/4"-1" wide cool each weld before going to the next.Butt welding works well with gas and old cars.mig weld is harder to hammer and dollie,its a much harder weld.

I think I can get to the back with a file. Ill have to pick one up. I only have a coarse round file at the moment. Right now Im just working on a smaller trim whole not a full patch. Im going to try a backing plate next time.

I filled in some tirm holes today. Getting my LaSalle ready to paint this winter I have a loing list of stuff to do. One of them was to bite the bullet and not look back and cut off the hinges that hold the hood sides on. No looking back now. I used a tig and some hole punches to fill the trim holes, I will post some pics of the process I did.

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Someday mine will look that nice... needless to say Im going to need filler for mine.

Why do anything to the backside? What is the drawback to having a proud bead on the inside of a piece of sheetmetal that you can't see or get to to grind? If it's that thin, you will probably do more damage to the surrounding steel right near the weld line by trying to blend in the weld. I'll bet you can't grind JUST the weld, but wind up grinding the high spots on the thin steel even thinner.

Hammering on a cold MIG weld is an exercise in futility, it's always harder than the deep draw steel they stamp body parts out of. All I've ever done is make a mess of the surrounding steel. Yes I have done both, butt welded with a mig as well as a TIG, and you can hammer a cold TIG weld flat if you can get to it. I do all my sheetmetal work with the TIG now, just because the finish work and warpage is so much less.

If I do have to MIG weld sheetmetal, I use a copper backing plate, that keeps the burn through and boogered up backside to a minimum.

Myself, I wouldn't worry too much about it if you can't get to it, but that's just me. I notice that the finishing welds link says nothing about the backside....

If it really bothers you, cut out the inner panels so you can get to the back of the weld to metal finish them, then weld the inner structure back in.

Later, mikey

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WOW!! six gun ,I can tell by that brake and your table that your serious about your work Do find it hard to prime your work? I do. I hate covering it up.Like mike says I wouldnt worry to much about the inside or for that matter the outside ether you'll be grinding it down and bondoing it up anyways.Those pretty welds will only come with experiance.and a fairly good welder.Keep at it you'll get there.

Be carefull to grind only the weld and not the surounding sheet metal grinding disks dont work very wel and heat things up to much Plus they're very expensive.I've found the bestway to grind only the weld with complete control is with a grinding stone on a die grinder,as they wear down they can get more and more usefull for tight spots , plus they last forever I keep afew on hand that are differnt sizes,aHF die grinder runs $5.00 the stone is even less.the tool on the far right is the one I'm talking about...

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Yeah, Ive got one, but never had much luck with it. Ill have to look for that style of stone as I haven't seen it before. I had good luck with a worn flap disk this time and a bastard file worked well to get it close to flat.

Another follow up question related to the back side of the welds. What should I do to clean them and then seal them from moisture?

Here is some of my work, I added 5/16 in height to a brand new Rootlieb hood by cutting and welding in a 5/16" strip the length of both sides of the hood.








Tack and hammer..tack and hammer...tack and hammer...



Each one of those sharpie marker marks is 1/2" apart



This was before I added filler, no place on the hood took more than .020" of filler.



Here's a thread about it.
http://www.hotrodders.com/forum/sec...hood-91241.html

Here's a thread about making a punch to make slugs to fill trim screw holes.
http://www.hotrodders.com/forum/econo-punch-92753.html

Later, mikey

__________________
my signature lines...not really directed at anyone in particular..

BE different....ACT normal.

No one is completely useless..They can always be used as a bad example

Yep, I really wish I had a booth. may get one someday, have to cover everyting up, have to clean up everthing afterwards, have to mop the floor 100 times so the pets dont track around in it. have a 55 plymouth sitting,the front fenders are bare, been two weeks now and they are not in primer because its a pain.

when grinding, I turn the speed way down on the air tools, way more control
that way. when I weld in plugs I use a unibit and just start to cut with the next size, leaves a nice little chamfer for the weld to lay, like mike I use a tig and if I mig I use a copper backup so there is nothing on the backside.

I have never done any metal work on newer stuff, one of my sons welded in the fuel door on his 64 Chev, that thing had so much tension in it could never get all the oil cans and wrinkles out of it.