This is one of the most common uses for a MIG welder, butt welding thin bodywork-type sheet metal, normally for a classic car restoration or repair.
This type of welding can also cause a lot of headaches and is actually one of the hardest welds to perfect, I for sure haven’t perfected it yet!
Butt welding is the type of weld where the edges of the sheet metal meet up with no overlap. This type of weld is very useful for body word repairs as once the weld is finished it can be ground down flush with the metal work leaving an invisible repair.
The main issue with thin body work is the fine line between correct penetration and blowing holes, and believe me its a fine line! Alongside this we also have to deal with the issue of distortion, which tends to happen if you get too much heat into a large flat panel.
Step 0: Panel Fitment
(Not to do with the weld per se but an important step)
If you are new to welding you may wonder ‘How closely do the panels need to fit?’
I know I wondered this when I first started and its one of the most important steps to achieving a good final weld. I typically leave not much more than the thickness of the sheet between two panels, so in the case of car body work this is typically less than 1mm.
Perfectly fitting panels can be achieved but you must question if that is really what is needed, as I will mention later the small gap between panels can actually help due to expansion/ contraction when welding.
But the general gist is to get a fairly close fit (sub 1mm), and if possible a nice even gap as well as it will allow your welding to be more consistent.
Step 1: Tack Welding
In my eyes this is the most important step! If you rush and tack weld the panel in the wrong place (not flush for example), then it doesn’t matter how good the actual weld is the repair will never be invisible. I typically tack every 15-20mm along the panel, often going back to adjust the panel between tacks.
Where you start and in what order you tack is situation dependant but I often do both ends first on flat panels, but on curved panels sometimes it helps to work across from one side to ensure the panel matches up over the whole section.
Step 2: Welding
The most common way for MIG welding thin sheets is to use what is known as ‘Stitch Welding’ as consists of a multitude of individual circular welds laying on top on one another, as shown. This method takes longer than normal welding but will help to prevent blow through and putting too much heat into the panel. It also looks quite neat if done correctly.
To prevent warping its good practise to do small sections at a time then move to other parts for the panel to prevent too much heat build up in one area. Alongside this, the gap present before welding can effect warping in the panel. If the panels are physically touching there can be a tendency to ‘push’ the panels apart during welding, most noticeably during tack welding where there is nothing to prevent the panels moving. Alongside if the gap is quite large the weld can ‘pull’ the panels together.
You can see the setting I used and my first section in the images.
Step 3: Finish Weld
As per Step 2 you continue welding all sections, taking care to alternate the areas to prevent heat build up, until the whole weld is complete.
Its worth talking about weld penetration here too, as you must check the backs of the welding to ensure that correct penetration has occurred, if you can still see the lines on the panel edges then the penetration is inadequate, you can see an example of this on the rear of one of my tack welds, proving that I still have much to learn!
I find the best way to grind down a completed weld is to use a flap disk mounted in an angle grinder, these disks are slightly flexible so tend to leave a much nicer finish than a grinding disk or burr. They are quick to use, cheap and leave a good finish. Typically I would use a 60-80 grit wheel to flat down welds, although a 100 grit is sometimes used to get a better surface finish dependant on location.
Any questions, comments are recommendations for further articles let me know in the comments below!