Hey everyone, if you’ve ever worked with sheet metal fabrication—whether you’re a engineer, a small business owner sourcing parts, or someone just getting their hands dirty building stuff—you know that welding is the make-or-break part. I’ve been running a sheet metal fabrication shop for 7 years now, and I’ve seen every single thing that can go wrong when heat hits thin metal. Last week we wrapped up a 500-unit order for custom HVAC panels, and 3 of the welds failed our final QC check—all because of a common defect we could’ve avoided if we’d caught it early. That’s why I’m breaking down the most common welding defects we run into with sheet metal, how we fix them, and what you can do to prevent ’em too. No stuffy textbook jargon, just real shop floor stuff I’ve learned the hard way. Sheet Metal Fabrication
First off, let’s remember: sheet metal is different from thick steel. It’s usually 1/4 inch or less, sometimes super thin at 16 gauge (that’s like a soda can, but sturdier), so you can’t crank the heat like you would on a trailer frame. That’s where most of our mistakes happen—we forget we’re working with thin material, not heavy plate.
Let’s start with the defect we see more than any other: burn-through. Dude, burn-through is the worst. It’s exactly what it sounds like—you heat the metal so much that you create a tiny hole right where you’re welding, because you burned straight through the sheet. Last month we had a rookie welder who used the same amperage he uses for 3/8 inch steel on a 14-gauge enclosure panel, and boom—12 holes in a single day. Fixing that? We had to grind them out, add filler, and re-weld, which ate up 4 extra hours and almost made us miss a deadline. How do you prevent it? We tell all our guys: test your settings on a scrap piece first, even if you’ve welded the same thickness a hundred times. For thin stuff (16 gauge and up), we drop amperage to 80-120 amps (for MIG, which is what we use 90% of the time), run a faster travel speed, and make sure the gun is at a 15-20 degree angle, not straight on. Also, back purging works for some alloys, but for most standard steel, just making sure you’re not holding the gun too long in one spot is key. Pro tip: we keep a stack of scrap sheet metal offcuts by every weld station, so guys don’t have to go hunting for test pieces.
Next up: porosity. That’s tiny little bubbles or holes inside the weld, like Swiss cheese. You might not see ’em when you’re welding, but once you powder coat the part or pressure test it, those bubbles become leaks. We had a customer send back a set of custom water tank brackets last year because 2 of the welds leaked when they pressure tested at 100 PSI—turns out there was porosity inside. What causes this? A bunch of stuff: dirty metal, moisture in the gas line, or using bad filler wire. We’ve had porosity from just storing wire in a damp garage, y’all. How do we prevent it? First, always clean the area around the weld with a wire brush or acetone—no oil, rust, or mill scale there. We make sure the gas line has a moisture trap, and we check the gas flow rate every morning (it should be 20-25 CFH for MIG). Also, don’t touch the filler wire with your bare hands—oil from your skin gets on it and causes bubbles. We keep gloves on at all times near the weld station, and we only use wire that’s stored in sealed containers. If we open a spool and it’s been sitting out for more than a week, we bake it in our wire oven for 2 hours at 250°F to drive out moisture. That’s a quick step that’s saved us so much hassle.
Another big one: incomplete fusion. That’s when the weld metal doesn’t actually melt and bond with the base metal, like two separate pieces sitting next to each other. It might look fine on the surface, but tap it with a mallet and it’ll snap right off. We caught this during a QC check on a set of heavy-duty shelf brackets last quarter—luckily before they shipped, but still, that would’ve been a disaster for the customer when their shelves collapsed under 500 lbs. What causes incomplete fusion? Usually too slow travel speed, or the gun angle is wrong so the arc isn’t hitting the base metal properly, or we’re not putting enough filler metal in. For sheet metal, since it’s thin, if you hang around too long in one spot, you might only melt the top layer and not the actual base metal. How do we fix this? We tell our guys to keep the gun moving at a steady, quick pace—no pausing mid-weld unless you have to. Make sure you’re angling the gun so the wire is pointing at the edge of the joint, not straight down. And don’t skimp on filler—even thin sheet needs a little extra wire to make sure it fuses. We also do visual checks with a magnifying glass on every weld, just to make sure the edges are blended in, not sitting on top of the metal.
Then there’s distortion. This is the most frustrating one, because it doesn’t show up as a hole or a leak, it’s just the part twisting or warping after welding. We had a customer order a set of 4×2 ft mounting plates, and when they came out of the welder, they were bowed so bad you could roll a marble across them. We had to re-cut 12 of them, which cost us material and time, and the customer almost went to another supplier. Distortion happens because when you weld, you’re heating a tiny spot of metal really hot, and when it cools, it shrinks. Since sheet metal is thin, it shrinks a lot more than thick plate, and that warps the whole part. How do we prevent this? Clamping is non-negotiable. We use magnetic clamps, C-clamps, and even fixtures that hold the parts in place while they cool. For long seams, we do tack welds every 1-2 inches first—don’t weld the whole thing in one pass. That way, you’re not putting all that heat in one spot. Also, we use a technique called back stepping: weld from the center of the part out to the edges, so the shrinkage pulls evenly, not to one side. For super thin stuff (18 gauge and up), we even use a tacks only first, then come back and do a quick stitch weld, so we don’t overheat the whole part. Trust me, taking an extra 5 minutes to clamp and tack is way better than having to re-cut a whole part.
Oh, and let’s not forget about spatter. Spatter is those tiny little blobs of melted metal that shoot off the weld and stick to the sheet. It’s not a structural defect, but it’s a pain in the ass—you have to grind it off, and if you don’t, it will flake off later and mess up paint or finishes. We get spatter mostly when we’re using wrong gas settings, or dirty nozzles on the MIG gun. Last week, one of our nozzles was clogged, so spatter was flying everywhere, and we spent 20 minutes grinding off parts of a small enclosure. How to prevent spatter? Keep your gun nozzle clean—we clean them with a wire brush every hour, and replace them when they get worn. Check your gas flow (low gas causes spatter too), and some guys use anti-spatter spray on the nozzle, which works great. Also, make sure your wire feed speed matches your amperage—if they’re not balanced, you’ll get way more spatter. We have a cheat sheet by every station that lists the right settings for each gauge, so guys don’t guess.
Wait, what about undercut? That’s when you have a little groove melted into the base metal right next to the weld, like a dent. It weakens the joint, and we’ve seen it cause parts to break under load. We had a set of truck bed floor panels that had undercut on the edges, and when the customer dropped them off a lift gate, the edges cracked. Fixing that meant grinding out the undercut and re-welding, which slowed down their project. Undercut happens when you’re welding too fast, or amperage is too high, or the gun angle is wrong (like too steep, so the arc is blowing away the base metal). To prevent it, slow down your travel speed a little, lower amperage if you’re on thin sheet, and keep the gun at that 15-20 degree angle so the arc is directed into the weld joint, not the sides of the base metal. We check for undercut by running a fingernail along the weld edge—if it catches, that’s undercut we need to fix.
Now, I know a lot of people think these defects are just part of sheet metal fabrication, but they’re totally preventable. The biggest mistakes I see (and made, early on) are rushing through setup, not testing settings, and cutting corners on prep. When we first started out, we’d skip testing on scrap and just weld the part, and we paid for it with rejected orders and unhappy customers. Now, our QC process starts before the weld even happens: a pre-weld check where we verify material thickness, clean the joint, and test settings on a scrap piece of the same material and gauge. Then, after welding, every part gets a visual check, and 10% of them get a quick pressure or bend test to make sure the weld holds.
Another thing: training. All our guys go through a 2-week training program just for sheet metal welding, not just general welding. They practice on scrap for 5 days before touching customer parts. We also do weekly “defect huddles” where we go over any rejects from the week, talk about what caused it, and adjust our processes so it doesn’t happen again. Last month, we had 2 burn-throughs in one week, so we dropped our amperage on 16 gauge by 10 amps, and we haven’t had one since. Small adjustments make a huge difference.
At the end of the day, sheet metal fabrication is all about precision, and welding is the core of that. If you’re sourcing parts for your project, don’t be afraid to ask your supplier what their QC process is, or what steps they take to prevent common defects. A good shop won’t mind walking you through it—we love talking about this stuff, because it means our customers trust us to do the job right. If you’re working on a project right now and need custom sheet metal parts, or if you’ve had bad experiences with weld defects before, hit us up to chat through your needs. We’re always happy to share what we’ve learned over the years, or take on your next order and make sure it’s welded correctly, no defects, on time.
Sheet Metal Fabrication References:
- American Welding Society. (2020). Welding Defects and Their Prevention in Sheet Metal Fabrication. AWS D1.1 Structural Welding Code – Steel.
- Lincoln Electric. (2022). Welding Thin Sheet Metal: Best Practices for Amperage, Travel Speed, and Distortion Control. Lincoln Electric Manufacturing Guide.
- Miller Welders. (2021). Common MIG Welding Defects and Troubleshooting for Sheet Metal Applications. Miller Technical Bulletin #MWT-107.
- Fabricators & Manufacturers Association International. (2023). Quality Control Protocols for Sheet Metal Welding Operations. FMA Shop Floor Best Practices Report.
Mid (Dongguan) Intelligent Manufacturing Co., Ltd.
Mid (Dongguan) Intelligent Manufacturing Co., Ltd. is one of the leading sheet metal fabrication manufacturers and suppliers in China. We warmly welcome you to buy sheet metal fabrication for sale here from our factory. All customized products are with high quality and competitive price. Contact us for quotation and free sample.
Address: No.22, Jiaoping Road, Tangxia Town, Dongguan City, Guangdong, China
E-mail: bruce_qin@bishenprecision.com
WebSite: https://www.bishenprecision.com/
