The real trick to welding thin metal is to use a wire feed. Just kidding, this tip is to explain how to do it with a stick welder. A wire feed or tig welder is ideal for sheet metal but, we will assume that you don’t have one. We need to establish what is considered thin metal. There is no absolute “according to Hoyle” answer but, I always considered anything less than 3/16″ to be thin. So from this point on we are talking about less than 3/16″ material. The first thing to figure out is what kind of rods and what size of rods to use. If you have nice new 1/8″ material, I would go with 3/32″ 7018 rods. If you have old rusty painted up 14 gauge, I might be inclined to use 1/8″ or 3/32″ 6011 rods. You might ask me, why don’t you just grind the rust off? Well if it’s just surface rust go ahead, but if it’s pitted out you will grind all the material away before you get it clean. Not only that, but if you are just doing a quick repair job on something, just weld it. Chances are, if the material is that bad the weld will far out last the rest of the material anyway. Back to my point, 6011 is the rod I will pick anytime. It allows you to move a lot faster than 7018’s will.
As far as the actual welding goes, I like to use a whipping motion with the welding rod following the same direction as I am welding. Basically just a back and forth motion. The reason for this is to keep from building up too much heat all in one spot. If you were to drag the rod slowly you will have a pretty good chance of burning through. If you are welding vertically, don’t be afraid to run the 6011’s downhill. If the material is really thin, I will use the whipping motion downhill as well. I know it goes against some principles of burying slag but, when the material is thin it really doesn’t matter as much. Most anything that is made from thin material is over welded. Say for instance you are patching an air compressor tank that has a hole rusted in it. You will weld the patch 100% because it needs to be air tight, not for strength. If you are making a section of air duct, you will weld it solid for air and not for strength, you get the point.
I guess the moral of the story is you can do just about anything with a stick welder in a pinch. It only takes a little bit of practice to make it happen. Just grab a piece of sheet metal to get the heat set correctly and go to town. A good heat setting for 1/8″ 6011 rods on thin material is about 80-90 amps. For 3/32″ 7018’s it’s about the same. Everyone is different but, those settings will get you in the ballpark.