Welding body work - help/advice?

[AaronKP30]

Hi all,
Been lurking for a while, and now need some advice.

I have been MIG welding repairs on my own cars for years - chassis and a bit of bodywork, usually with reasonable results. However, I need to repair a flat boot skin and am having trouble.

My own PC has died, so no pics, sorry! I'll try and describe the job.
If you can imagine a flat rectangular skin about 4' across the car by 1.5' front to back with no features other than a 1.5" downturn at the rear edge.

The repair is about 3" by 2" and about 0.5" above the downturned edge. When I made the repair patch, I made it a close fit in the hole. When tacking in the repair, the panel started distorting. I was getting good penetration, and taking my time, alternating around the patch and letting it cool. MIG is a Sealey 150A set to 2nd of four power levels. Wire is 0.8mm.

I have cut it back out and will be trying again Saturday hopefully.

Any hints would be appreciated.

Thanks,
Aaron.

 

[peterd51]

Hi,

possibly too close a fit?

As you put the first tack in it maybe moved the patch over very slightly?

I'd prefer a slight overlap on a job like that, maybe 1/8" all around. As I melt the edge of the the new patch it joins the exising metal. There's not much of a gap left for water to get in and it's sealed underneath anyway.

Regards
Peter

 

[AaronKP30]

Hi Peter,

Thanks for the reply.
Do you mean to form the edges of the repair section so that it looks something like this

---------
_/ \_


And then bring it up from under the old panel so the ------ surface is flush to the panel being repaired?

Regards,
Aaron.

 

[AaronKP30]

That diagram above didn't format right.

It meant to show each edge of the repair being stepped down by the thickness of the metal so it could be set against the underside of the original panel.

 

[Morrisman]

No matter what prep you do you'll end up with distortion, welding in the middle of a flat panel of sheet metal. You can hammer the welds to re-expand the metal and take the distortion out. You'll need a dolly at the back to beat against, then light tap on the weld around the circumference and the distortion will disappear.

well, that is the theory, but it is never quite so straightforward in real life. mig welding is a bit too hard to hammer successfully too, but it works to a degree.

Making the patch rounded instead of angular helps too.

And cutting it out and welding another in will probably make the whole thing even worse, as you have already distorted the sheet of steel.

 

[peterd51]

Hi,

You could put a flange on it using a tool called a Joggler, but to be honest I never bother.

A lot of guys here like to weld the new piece in flush with butted joints and everything ground down so you can't see it. If it's a high status vehicle or restoration of a classic then I can see why people would want to do that, but I've not done it in the past.

Also, a lot of guys argue that even if you leave a small overlap then damp will get in and rust and again I can see their point.

I tried a flush/butt patch on my most recent repair and I wasn't impressed, it looks bloody awefull especially from underneath and I didn't really have time to re-weld it.

So I'll stick with my old method and just lay the new bit over or under the old bit and weld it in. If it needed to be flush then I'd fill it but generally I don't work on anything that special.

Regards
Peter

 

[AaronKP30]

Thanks guys,

This is a semi-restoration but will be used regularly on the road when finished. The car holds sentimental value to me and the body is generally in good order so repairs will need to look good to me or I'll be annoyed at not doing a better job.

When I cut out the distorted repair, the flat panel sprung back into pretty much the original shape. If I can get a repair seam-welded and ground down I don't mind a skim of filler, but the distortion was the worst I've ever seen - filler would have been over 0.5" thick.

I am tempted to try the "joggled" method and see if it gives better results.

Failing this, I am in the process of making glassfibre moulds of the panels as parts are no longer available, so a temporary repair to allow a mould to be made would do.

Aaron.

 

[Paul.]

If you can get to the back of the repair, back up the joint with a copper heat sink, you can also buy a paste to surround the joint , resistance spot welding is another option if you can access both sides of the panel, a bit of planishing is inevitable, if its only a cosmetic repair weld as little as possible and fill it, coldfront paste from www.frost.co.uk/productList.asp?catID=33&frostCat=Welding
Paul

 

[AaronKP30]

Thanks for the tips everyone. I finally got the chance to try again today.

Made the repair piece smaller so there was about 1mm clearance all round, clamped a block of aluminium behind panel and really took my time, 4 spots at a time, waiting until it was cool to the touch before doing another 4 spots and so on.

Much less distortion this time, should only need a skim of filler over the area after grinding back the welds.

Apparantly a flap wheel is better than a grinding disc - what grit flap wheel will be most effective without causing problems?

Aaron.

 

[Paul.]

Hi
glad your beating it! Flap discs tend to give a finer finish than there equivalent grade in ordinary sanding discs, a 60 grit flap will be something like an 80grit sander, i would go for that for finishing, and a 40 grit for heavier stock removal.
Dont press too hard as the heat generated can distort the panel too.

 

[Rich.C]

Hi Aaron, what is the minimum amps on your welder? you may be better off with 0.6mm wire and the lowest power setting for bodywork.

As Paul says go easy with the flap disc, you can soon make 0.8mm metal extremely thin (talking from experience )

 

[AaronKP30]

Rich C. - The welder just has Min/Max and 1/2 to give four power settings. I'm not sure of the current ratings at each setting.
I'd never had problems with major distortion before, I have just replaced inner and outer sills, rear arches and repaired the doors on another car with no problem, but these areas all had swage lines or some sort of shape that may have helped prevent distortion. I have to repair the door bottoms on this new project so may invest in 0.6mm wire and see if the lowest setting will be better as they are flat panels like the boot lid.

Peter - thanks for that, will order some 40grit for initial grinding back.

Aaron.