A common method of attachment for classic car body panels is the spot welded flange, unfortunately most people don’t possess a spot welder as these are quite expensive and less versatile than the more common MIG welder.
In this case the best option is to plug weld.
Plug welding is fairly easy but requires more prep than a spot weld would, and generally takes a lot longer, but will suffice if a MIG or TIG is all you have.
The first step is to prep both panels for welding, cleaning off all remaining paint, dirt etc.
Secondly a hole must be drilled in one of the panels, I typically use an 8mm hole, or 10mm depending on the flange width. Holes are typically spaced about 25-30mm apart, or what ever suits the panel you are working on. A thicker sheet will typically require a large hole.
Spot welded panels are notorious for rusting, the reason for this is quite obvious: water gets into the seam due to capillary action and can’t dry out again, leaving no option but to rust from the inside out. Therefore it is always worth considering putting ‘weld-thru’ primer on the flanges to be welded together to help protect the metal within the joint. Although I always clean the paint off the area to actually be welded so it does not interfere with the weld.
Before welding the panels must be clamped together tightly as the plug weld, unlike a spot weld, does not push the panels together. Clamping can be achieved in a manner of ways, my favourite being self tapping screws, mole grips or a plug welding clamp. The tighter the panels are together the better and often a mixture of clamping methods will be used at once.
Next, the step where the magic happens: actually welding the panel. Typically one would use a setting that would be adequate for welding a single sheet 1.5x the thickness of the two sheets to be joined. I normally start pointing the MIG torch directly into the centre of the circle onto the back sheet and keeping the torch here until the hole is almost full of weld (thus ensuring correct penetration onto the back sheet), then move around the outside in a circular motion to join the pool to the front sheet.
If all has gone well you should not be able to see the hole anymore and there should be evidence of suitable penetration on the back sheet. If not, try again with a higher current setting.
The next step, if the weld is good, is to flat back the area, paint and seal. The best option regarding flatting back plug welds, or any weld for that matter, is the flap-disk. These magical items are a godsend when it comes to body repairs and allow virtually invisible welds. Typically for flatting back welds I use either a 60-grit or 80-grit flap disk.
Next the seam should be painted and sealed. People’s opinions differ regarding the order of things here but I prefer first applying a high zinc content primer to the bare metal to help keep corrosion at bay, and then applying a good brushable seam sealer to the joint to help keep moisture out, and covering the whole lot with whatever top coat you require.
At this stage, if all has gone well you should have a nice repair almost indistinguishable from the original that will hopefully last for years!
Some pictures borrowed from: http://www.mig-welding.co.uk/plug-weld.htm
More Pics to follow…
If you haven’t already, why not check out my How-To for Butt Welding thin sheet, you can find it Here.