Classic cars forum & vehicle restoration.

[rcx822]

I've been reading up on sealing seams using PU. My car is going for e-coating soon and will need the seam sealing. The seams I've seen are just overlapping joints that have been spot welded. Why do restorers seal these rather than just run over them with a welder? Is that too time consuming?

Also the e-coating company has said that I should "visit a speacialist" to have the seams sealed, whereas online articles look very simple to do it myself. Any reason to use a specialist? Is there more to it than just sealing the seams with PU and a caulking gun?

[ukdave2002]

I have seen them brazed or soldered, both of these methods will seal better as the metal will flow right into the joint with a capillary action where as a weld wont, they also put a lot less heat in to the panel so less of a distortion risk.

Dave

[colwyn500]

In order to achieve what you suggest, flanged or stepped, overlap seams like you describe would require welding on both sides of the joint and even when welded up, this would still leave a void between the two weld runs which would obviously be bare of paint internally and would give perfect conditions for regular condensation formation. There would be lots of potential for all sorts of distortion and the strength of the join might even be reduced. Best practice is to use weldable paint inside these spot welded joints to slow down corrosion.

Restorers should ideally make clean, butt joints when a piece of steel is "let in" to a panel where it was originally continuous metal but they usually replicate any butt joints. I don't think there are often overlapping joints on original equipment bodywork but stand to be corrected. When repairs take place on large, flattish panels, such as doors, restorers may choose to reduce distortion and compromise by overlapping and spot or plug-welding panels together.
basically, you want as little localised heat put into the panels as possible so I would follow the advice given.
Good luck...photos please!.

[D4B]

[colwyn500]

Excellent advice ....
Thanks Steve...goes on a bit though
Not sure what next week's sermon will be.

[ka]

Spot welding was introduced for a couple of reasons; not only is it quicker, easier and can be managed by robots (human or mechanical)on a production line, there is very little distortion that occurs when any metal is heated.
If you consider brazing the full length of the joint, the distortion will be quite impressive, and also you introduce a different metal into the mix allowing an electrostatic reaction to take place, that encourages the rust to find a home.
Mig welding helps reduce this by utilising the same steel as the base, so reducing the reaction, but doing little about the distortion.
As recommended, use a weld resistant paint against the surfaces, spot weld (the mig can be used for this rather than purchasing a dedicated spot welder), then fill the joint with a decent, non-setting sealer, there are quite a few purpose designed mastics available. I am also a big believer in Wax-oil, but that is a different thread!
_________________
KA

Better three than four.

[rcx822]

Ok that clears things up.

[Bitumen Boy]

[MikeEdwards]

In some positions, spot welds might be used specifically to control deforming in the event of a collision, and seam welding might alter how that occurs. It might not be relevant to vehicles of the age that are usually discussed on here, but I know that major manufacturers were doing crumple zone testing in the early 1970s at least.

I am concerned about getting some kind of sealant so that spot welded flanges don't start rusting out again. There is some truth that sealing both sides with a good seam sealer and regularly checking to make sure it's still in tact works well - I have opened up more than one such flange on my current restoration project to find that where the seam sealer is still good, there is bright shiny bare metal (in a car from 1976). It's where the seam sealer fails that the problems start. I've been using weld-through primer where I can, but I've also found that aerosol red oxide seems to cope with nearby welding just as well.

[ukdave2002]

[ka]

Gas versus MIG,?
I would not use gas for any external joint, the heat required to melt the metals and any possible filler rod will distort anything. MIG has the advantage of reduced heat therefore less distortion.
The Bodywork garage I worked at, in common with most others developed a technique that reduced distortion to the minimum. For example when fitting repair wheel arch sections, depending on the vehicle, we would grind everything clean, clamp in situ, and then weld a pea sized weld. Think of the joint to be welded in sections,for example a clock face going from 9 to 3. Weld (pea sized) at 9, then at 3, then at 12, then at 10.30 then at 1.30, and so on. this allows the weld to cool, and reduces the distortion to almost zero.
Different items to be welded will need a different approach, there is a thread repairing a 100e, where a section is fitted in a door, using an offset bender to provide an overlap, rather than a butt (which I would avoid in all situations), sills we clamped, and spot welded using the MIG, again weld at opposite ends infilling between the previous welds, there is no advantage in continuous welding in 99% of circumstances, only in repairs to chassis.
We would seal the joint with a non setting sealer (similar to windscreen sealer being rubber based), prime paint, and then hit it with wax oil to finish off the job, and ensure next time we had the vehicle in, it was not because our work had gone rusty.
_________________
KA

Better three than four.

[ukdave2002]

[colwyn500]

There's some guys on the internet..Americans especially, who are a whizz at what you describe Dave. But in the real world of self-taught restorers with limited time and resources, most of us make do with that dot, dot, dot welding by MIG.
I found it relatively easy to remove the surplus bead off welded body panels with careful use of a flap disc. I butt-welded wherever I let steel into a visible body panels except on doors.
Example:
_JAN8081 by peterthompson, on Flickr

[Old Wrench]

I'll just chuck in my twopennorth.

Spot welding was introduced in unit construction, simply because it was cheaper!

And because it saved the problems of bolting wings etc onto the main body structure.

In the good old days, wings, e.g., were mounted using a fibre continuous gasket and nuts and bolts. With washer naturally.

It was the USA which introduced Spire fittings and caged nuts: and when they rust, you have a nice long job: e.g. E Type Jag front tubular chassis section, mounted thru the bulkhead. Lots of fun.........

When I worked for Ford Europe in the mid-1960s, the assembly line welders often used to fail to line the jigs up properly: Cortina MK 1 floor pan to rocker panel (Cill) was a constant problem: in use the car used to "Groan" as badly fitted and welded panels use to work against each other over bumps!

Ford's quaintly named "Service Fix", by dealers was to spray copious amounts of AdSil into the joints! Subdued the noises.......

Decent cars such as Jaguar and Humber needed the wings seam-brazed and then lead filled.

Any rusting in service would emanate from the underside, either side of the joint: and sometimes, from the joints themselves if the brazing flux had not been fully removed; wire brush and boiling water.

I disagree about heat induced distortion. This only occurs when far too much heat has been used.

Any type of welding, be it TIG, MIG Shielded Arc, needs to reach the melting point of the parent metal (Smelt Point) to ensure good penetration and with mild steel this is circa 1,500 degrees C. Whilst mild steel is not a brilliant heat conductor, that's enough to cause localised distortion.

Welding also creates crystalline structures around the parent metal and the weld fillet; which cause inherent local weakness spots: good brazing doesn't need to raise the parent metal to a high enough temperature to cause this effect.

Indeed, when we built/repaired racing car space frames and tubular suspension components (typically fabricated wishbones, radius arms and etc, we always used Manganese Bronze filler rods as against Silicon Bronze. Since it goes in at a much lower temperature than Silicon Bronze.

This was then the standard approach, as the lower temperature reduced the possibility of joint fracture at stress points, thanks to continuous loading-unloading of the component and torsion effects.

Ford Competitions, at Boreham, used a particular type of silicon bronze rod to "Seam Weld" new body shells, for rallying etc. I managed to obtain the same rods from my Stubbs: much higher tensile strength than conventional silicon bronze rods.

Competition Monocoque -i.e. Unit Construction bodies - were Seam Welded to create a very rigid structure, to preserve suspension geometry and set up. Normally, mass produced spot and barrel welded bodies flexed so badly, they were hopeless).

Tips for oxy-acetylene brazing panels together:

1. Mix the flux with warm water:

2. Brush on lightly:

3. Use a slightly oxidising flame: i.e. more oxygen than normal neutral flame:

4. Use a very gentle flame, much lower than one might think:

5. Use a very thin filler rod: typically 1/16th inch:

6. "Chase" the filler bead of molten bronze along the joint, as quickly as possible,"blowing" the bead with the gas pressure.

7. Be totally scrupulous with cleaning the flux: when the work has cooled completely! Pouring water onto a hot joint will induce distortion: vigorous wire brushing finally is essential.

8. If you see hardened globules of flux then you used far too much!

(N.B. Edited for typos and clarity. OW)

[ukdave2002]

Hi old wrench

I think brazing is very underrated by many restorers, a lot less heat is involved and the equipment is much cheaper, as with gas you can really see what's going on

I braze quite a lot of stuff...

Dave