Welding sheet metal

This writeup is a guide to some common problems you encounter when welding and do steel work on cars. The focus is on the use of basic tools, expensive tools like English wheel is not covered.
I do not go to detailed into general welding technique, but there are heaps of that on the net.

Please read sheet-steel-work in conjunction with this post.

See Body Solder on soldering.
See door-skin-repair for example of welding a door.
Welding up front bib of a 40: land-cruiser-40-bib-restoration
Restoring windows frame: restoring-a-land-cruiser-40-windshield-frame/

Examples here are from a welding the window frame on a Land Cruiser 40 hardtop. Usually I solder smaller holes in non structural parts, but these are to big and has to be welded. See body-solder for soldering.

MIG welding

MIG welding is by far the easiest welding technique. You push a button, and the welding wire pops out, covered in an inert gas to avoid oxidation. Thick steel is unproblematic, but it is not that easy when it comes to thin sheet metal.

Steel work

When welding you need steel parts. Some times you can purchase new parts, but mostly you have to fabricate your self. I will cower how you can make simple parts without any expensive equipment. To make really professional parts you will need an English wheel, planishing hammer, bead roller etc which easily sums up to thousands of USD. But you can do a lot with simple equipment.
See sheet-steel-work

Welder

If you are going to by a welder you must think of your future use of it. If you are going to weld thick steel, say 4 mm and thicker, you need a lot of ampere. At least 180A. For sheet metal welding only a small welder is sufficient. I would go for a larger one, and maybe a used professional welder.

I bought this well used 250A Esab Smashweld 10 years ago and have had no problems with it. I use Mison 18 gas (Argon with 18% Carbon dioxide), and 0,8 mm thick wire.

When starting welding you should properly test it out, and maybe start on a project involving thicker steel. My first projects where a transfer case skid plate and a rear bumper for my Land Cruiser 80. See posts: rear-bumper-land-cruiser-80

The adjustments on a welder can differ a lot, but it is actually only two main adjustments: current in Ampere, and wire feed in inches or cm per second. You will find a table on your welder or in the documentation suggesting what ampere setting for a given steel thickness. The feeding must also be set. But these settings are only suggestive. Test out before starting.

To get a good weld it is important to use highest possible current without burning holes in the material. This will assure a good through-weld. Start with a high current and adjust down if necessary. By adjusting wire feed you can also control the weld. A higher speed means more wire and less chance to burn holes. On the other hand, to much wire will make the welds high, and you must grind more. This does not matter on heavy steel profiles like a bumper, but is to be avoided on thin steel.

Tools

For welding you need pliers of the locking Wise-Grip type, magnets and maybe butterfly clamps/intergrips if you work on large panels. Also ordinary carpenter clamps are handy. And various hammers and dollies.

Welding sheet metal

Typical thickness of the panels of a car is 0,9 – 1,2 mm. Also thinner on newer cars.

When you are joining two panels there are two techniques, butt-weld and overlap. When you butt weld the two panels are joining flat against each other and you weld them together in a continues seam. With overlap welds you put one panel above the other in a an overlap, usually some centimeters wide, and stitch them together point wise with say 2 cm distance.

Before starting any welding clean away all paint and rust thoroughly.

Warping

If you get warps you can try to hammer it out. This will cause stretching of the material. If you want to shrink the material the process involves heating with a torch. See below on oil-canning.

Overlap weld

Overlap welds are faster as you don’t have to cut the patch very accurately, and faster welding witch is less prune to warping. The drawback is that you have to use a lot of filler, and the joint is prone to rusting in the joint.

Butt weld

With a butt weld you have to cut the patch rather precisely, see below on making patches, and welding will take more time, as you weld the whole joint. The benefit is that you get a smooth surface, and if properly done minimal filler has to be applied.

When you have cut the patch use pliers, magnets or butterfly clamps (see below on tools) to hold the patch in place at same height as surrounding material. If it is not possible to get it flat all the way around start in one place and weld it there, only one short burst to make one point. Make sure there is not a perfect fit. Allow around one millimeter gap between the patch and surroundings. This is to prevent warping when the steel expands. It also gives a better trough-weld. If the opening is more you should move the welder a bit from side to side while welding to fill the gap.

Then work your way around the patch, move pliers if necessary, and place a spot-weld with 4-5 cm distance. After that you can start filling in the room between the spots. Only weld one and one spot at the time, maybe 3-4 spots and allow the steel to cool down. This is very important to avoid warps. To avoid this to take too long time use compressed air, if you have, to cool down. The steel cools down in a few seconds, and makes the welding far faster.
If you get small gaps in the weld don’t try to fill it as you will only build up the nearest weld. Go over en weld all holes after grinding.

Also important is to hammer out the weld at regular intervals. Because of the heat the metal will shrink around the weld, making the weld to sink, and strain is introduced. By using a dolly behind the weld and hammer it you flattens the steel again and removes strain. It does not matter what side you hammer from. When working on a closed cavity hammering is not possible and you have to use filler instead to fill the sunk area.

Grinding the weld

If you are using enough current and not to much wire feed the weld should be rather flat. This is not always the case as sometimes you will miss the gap and weld on the side, and when welding it again you will get a high weld. Also where you have burned a hole and filled this you will have a blob.

Anyway, you need to grind the weld down. Use a small grinder with 10 or 12 cm grinding wheel. When grinding try not to remove more steel than necessary. If there are sunken areas use filler instead. Do not grind to long in one spot, but work around. Else the steel will become to hot. If it becomes blue it is hardening.
After grinding you can go over with a flapper disc to smooth it out. I also use a sand blaster before painting. See painting-and-rust-protection on painting.

Spot weld

To remove spot welds you use a special drill bit, see picture.

When you want to make spot welds you drill a hole 6-7 mm in diameter, preferably in the non-visible panel. Then you set the welder to twice the current you use for the panel thickness in question, point the welder to the center of the hole and shoot. Repeat until hole is filled. Be sure to press the panels together.

Fill holes

When you weld thin steel you will burn holes in the steel, especially if there are rust in the area. If you never burn holes you are running on to low current. But holes are no crisis as they can be filled. To fill the hole weld at an inclined angle towards the already welded part of the hole. Then in short bursts fill the hole always working against the welds where the steel is thickest. If necessary reduce current.

Door sheet welding – oil canning

I welded up lower panels on the doors on my 40, trying to follow the book: weld a few spots, cool down with compressed air, hammer out the weld. And so on. See door-skin-repair.

Still I got some stretching of the metal in the left corner, giving the so called oil can effect, where you can pop the steel in and out.

Oil canning is a well known effect due to shrinking where the edges move inwards when you weld large, flat areas.

But there is a cure; you can shrink the oil can. This can be done by heating spots of the affected area, gently hammer it, and cool down with water. Heat until nearly red, hammer gently against a dolly to shrink, and then cool down using a wet rug. Start in the center of the oil can and move outwards in circles until the oil can have gone. It is easy to warp the sheet in the process so be careful. And there always is filler…

Do not try to fix an oil can with filler. Due to the movements of the steel sheet it will crack up. If you have to, use body solder instead. See Body Solder.