Practical MIG welding - Fabricating complex shapes

Often parts of old car bodywork such as wings are no longer available or are expensive. Normally they only rust around the edges, so can be repaired, but the complicated shapes can be daunting.

Here's a method of repairing these parts that doesn't rely on panel beating skills.

Steel behaves in much the same way as card - it's easy to bend in one direction, but you'd need a press or some idea about how to panel beat to make a compound curve.

It might have been quicker in this case to make a new bumper from scratch rather than messing with this old one. The way to do this would have been to bend a length of thin card (pizza box) into a U shaped channel, then make V shaped cuts in the sides to allow it to form the shape of the bumper. Once the card was in the correct shape it would be flattened again and used as a template to cut out a new piece of sheet steel which would then be bent into shape and the Vs welded.

Instead I followed a slightly less elegant approach:

rush hole in a bumper

The first step was to build some stiffness and reference points into the bumper. The first repair section was just a piece of steel with a wide radius bend (to match the bumper) replacing a rusty section I'd removed with an angle grinder.

The new steel will need to go around a corner, so I cut a V in the end and bent the metal into shape by holding the bumper upside down and hammering against a steel table.

new sheet of metal welded in place

With the new piece of steel in place and bent into shape I formed the shape of the next section in card, and made a couple of cuts so it would follow the shape of the bumper around it's various angles.

The card was used as a template for a new piece of steel which I bent into the right shape and tack welded in place. A set of 3 different shaped welding clamps (that you can buy from just about any hardware shop) were very handy for holding the unusually shaped bits of metal together for tacking.

It ended up looking quite complicated as if it had involved some thought, but actually once one the first bit was done the next was easy.

complex fabrication in metal

A third piece shoddily cut out and tacked to the other side completed the repairs. Grinding the tack welds flat and some more hammering with a soft faced hammer against a steel bench finished the shape off. The tack welds can occasionally break when hammering, but on the most part should stand up to the abuse.

It pays to have as narrow a gap as possible when MIG welding thin metal - that makes for an easier weld.

repairs completed

I did the seam welding quickly. Where I'd left a really small gap I could use a "back step" technique (weld for 10mm forwards (push) in a straight line, move the gun 20mm backwards, then weld another 10mm forwards until the new weld meets the start of the first weld. Repeat until finished. This technique reduces the heat build up and hence the chance of blowing holes.

Where I'd left a 1mm or more gap I used the stitching technique prescribed for thin metal on this site.

seam welding of joints

Here's the rear of the completed bumper. I've cut the old metal out and butt welded new metal in it's place, so there are no double skins to trap water (and hasten rust in the future).

repers from underneath

It's when grinding down the welds that you appreciate the time spent making the metal fit together. Where I'd made 2 sheets butt tightly against each other I'd been able to make a very quick and small weld which was quickly ground off. Where I'd left a big gap I ended up with a whole load of weld to grind away.

Still, a check with a straight edge, and a bit more hammering finished the metalwork.

welds ground down

Prior to finishing with body filler it's best to put a bright light behind the welds and check for pinholes of light showing through the weld. These would let water in from the rear and cause the filler to bubble. Any pinholes can be covered with a blob of weld and ground down again.

I tend to apply body filler with the normal plastic scraper, then use a long straight edge to wipe the excess off. It's a way to end up at the right shape quickly.

body filler applied

A bit of primer and then black paint, then a wait until it had started raining so the many imperfections were hidden from the camera, and here's the finished job.

Yes, it's that Maestro Camper again!

completed and painted bumper

This kind of repair (butt welding lots of different sections) is great for non-stress bearing parts. For a chassis you'd want to make a single panel and weld it in a similar way to the manufacturer.

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