Welding sheet metal patches

[Pop N Wood]

How much luck do you guys have MIG welding in sheet metal patches that look like unpatched steel?

 

I went to patch up some small and large holes in my firewall. I mananged to cut some patches and weld up the heater hose penetrations. Ground everything down and with just a little spot putty the patches should disappear.

 

But I was not so lucky when I tried to do the same thing to penetration someone else had cut for AC. I cleaned up the hole and cut a patch to fit perfectly. Initially had some trouble running a bead cause I didn't clean off the inside. Got that fixed up and still struggled a bit with my welder and not being able to see.

 

Anyway got done welding everything and ground the welds down. A bit rougher than the heater hose patches, but thought it looked OK. Turned all the shop lights off and went to leave. I had to go back and turn off the drop light I had left in the car. That is when I saw all kinds of light streaming through pinholes all around my welds. The heater hose patches were solid, but not the AC patch.

 

Went back last night and tried to reweld everything. But with all the burn throughs I was losing ground. Ended up sandwiching a small sheet metal patch over the flush patch.

 

So how about it. Is it unrealistic to think I should be able to grind a MIG weld down in sheet metal and have it disappear with no pinholes?

[JMortensen]

If you grind it down completely flush you're going to find pinholes. That's been my experience. You said "running a bead" and that is a mistake. What you should do is a hundred little spot welds around until you've welded the whole thing. Then leave it alone. Grind from the side that you want to look nice, and leave the welded side ugly, or if you do grind the welded side just smooth out the bumps but don't try to make it flush on both sides. That's what I'm doing.

[SHO-Z]

. You said "running a bead" and that is a mistake. What you should do is a hundred little spot welds around until you've welded the whole thing. Then leave it alone. .

 

To shave my side lights on my car I used a mig with gas purge. I welded about 1/2" long bead and let it cool. Also skip around on the filler piece to reduce warping. Then ground it smooth and did a little filling with flexable body filler. Running to long of a bead on thin metal will cause warping and you will burn through. I could care less what the back side looked like.

 

Take your time and do it safely!

[ezzzzzzz]

Jon has it right. Unless you have really clean metal and finesse it is difficult to fit in patches without it showing. I will cut the patch to fit tight. I then add a few small overhanging vertical pieces spotted onto the patch to hold it flush in the hole. I few spot welds will fix the patch (from the viewed side)and I remove those vertical pieces. I SLOWLY spot weld the whole patch holding the gun at a radical angle to reduce/eliminate burnthrough and heat buildup. I will then weld from the opposite side (unviewed) to build up the weld. Finally, I can grind down the viewed side close to, if not, flush with few or no pinholes. Some body filler and paint finishes the job.

[JMortensen]

One more thing for you Pop. I was considering trying to find some rubber plugs for the heater hose holes rather than try to weld. haven't gotten too far with that yet, but that's another option vs welding.

[johnc]

If you can flange the patch panel and place it from the inside of the car, the exterior part of the patch will sit flush with the rest of the sheet metal. You can spot weld it from the inside if you punch holes in the flange and reduce the chances of burn through or pinholes. Something like this helps:

 

http://www.eastwoodco.com/jump.jsp?itemID=1587&itemType=PRODUCT

[Guest TeamNissan]

One more thing for you Pop. I was considering trying to find some rubber plugs for the heater hose holes rather than try to weld. haven't gotten too far with that yet, but that's another option vs welding.

 

You could try home depot, I know they sell rubber plugs for fill the hole when you dont use or remove the hand spray from a sink. They make press in rubber ones and crome faced metal once that screw lock from the back.

[Sparks280zt]

You might want to PM 240hoke. He knows of a good place that sales tons of rubber products through their catalogs.

[G.I.jonas]

Welding sheet metal is not easy,what j said about a hundred spot welds is the best way to make it easier,however with the right welder and experience perfect beads are possible.It has taken years of welding to learn but i now have no problem running a weld bead on 22 gauge sheet without burning through however this is only ok when warpage doesnt matter.You should try one of those copper thingies you put behind your weld to prevent burn through and the weld doesnt stick to it,i think eastwood has them too.

[clint78z]

Some tips I have found to welding thin metal on the Z.

 

1) Do not use the galvanized patch metal you get at Autozone..., got to a sheetmetal place and by 20guage. The difference is night and day.

 

2) Surface prep edges put a little bevel on them very slight. Clean both surface throroughly with grinder.

 

3) Fitup is extremely important, I have better luck with a buttweld. Take the extra time to make a zero gap. Take extra 15mins here, it will make it so much easier. Make sure it is clamped very well, it's almost impossible to fill a gap.

 

4) Short busts, tack and go to the opposite side repeat multiple times.

 

Experiment on the bench where it's easy to work on with a couple of peices of 20 guage. Play with heat and wire speed. An aout darkening helmet makes it a lot easier tack welding thin material

[Pop N Wood]

Thanks for all the replies. I don't feel so bad now. Close to but not flush is now my rule. Guess I was trying too hard to make it perfect.

 

I actually have the heater hose penetrations done. They are built up a bit. I think having them right in front of my face helped alot. The AC penetration was impossible to get a good viewing angle on.

 

Yeah, I did it all with short tack welds. I used a water soaked rag to cool things down between welds and grinding to prevent warping. By not being able to run a bead I meant I couldn't get a short tack weld to go without popping. I think the tar on the inside of the firewall was coming through. That caused me to start adjusting the gun which started misfeeding making things more difficult.

 

I actually have pretty good luck filling in small holes or burn throughs. It is just in trying to grind them smooth that I got greedy. The flanging idea would have worked perfectly.

 

One other thing I have learned is a flourescent work light does not trigger an auto darkening helmet. It seems to put out a color of light that isn't as heavily darkened by the lens either. I can see way better with it than with a 500W halogen light.

[SHADY280]

the auto darkening helmut is the best thing to come to welding ever. what i even do, if i can, is weld the backside. but do it b4 you grind, that way there is something to grind against, and weld to if you spot a hole.

[mtcookson]

I'm working on a project right now that I have to deal with this and had a question... What if you don't have any access to the backside of the spot you're patching? Can you still spot weld the patch on and grind it smooth or is there another, better way of doing it? Its an exterior piece that will be painted once finished, I just have no idea what the best route of doing it would be.

 

Thanks in advance!

[JMortensen]

Grind it as close as you can and use a filler to make it all smooth. This might be the time to use the flanging tool that John Coffey mentioned if at all possible.

[mtcookson]

I think I may look into that. I read up on it a bit and now that I understand better exactly what it does that will definitely make it a little easier. Thanks!