How To Weld Car Sheet Metal
Content Menu
● Safety First: What You Actually Need to Wear and Do
● Essential Tools and Equipment
● Resistance Spot Welding in the Real World
● Dealing with High-Strength and Galvanized Steel
● Frequently Asked Questions (FAQ)
Introduction
Working on car bodies means dealing with thin steel that doesn’t forgive mistakes. Most panels are 0.7 mm to 1.0 mm thick (roughly 20–22 gauge), and the metal has to stay flat, rust-free, and strong after you’re done. Over the years I’ve patched everything from daily-driven Civics to show-quality Chevelles, and the same lessons keep coming up: heat control, fit-up, and choosing the right process decide whether the repair disappears under paint or haunts you forever. This article walks through the practical side of welding auto sheet metal the way it actually happens in garages and small shops, with settings, sequences, and tricks that work on real cars.
Safety First: What You Actually Need to Wear and Do
Start with a helmet that darkens fast—Lincoln 1740 or better. Cheap fixed-shade lenses leave you flashing yourself every time you strike an arc. Leather gloves that reach the elbow stop the hot balls of spatter that roll down your arm. A cotton shirt soaked in sweat will catch fire; wear a flame-resistant jacket or at least a heavy leather apron.
Zinc fumes from galvanized coatings give you metal-fume fever—feels like the worst flu of your life the next morning. Grind the coating off the weld zone and run a strong fan or fume extractor. Keep a charged ABC extinguisher within arm’s reach; I’ve put out two small fires caused by slag landing on cardboard. Disconnect the battery and remove the fuel tank or lines if you’re anywhere near the back of the car. Simple rules, but ignoring them ruins weekends.
Essential Tools and Equipment
A 140–200 amp MIG is the workhorse for 95 % of body work. Machines like the Hobart Handler 140, Lincoln Power MIG 210 MP, or Miller Multimatic 215 all do the job. Use 0.023–0.030 in ER70S-6 wire and 75/25 gas. A 20 cu ft bottle lasts months in a hobby shop.
For spot welds you need either a decent portable spot welder (Eastwood, Harbor Freight Titanium, or a used Tecna) or learn to make plug welds with the MIG that look factory. TIG is great for aluminum and stainless, but most steel body work is faster with MIG. Add a good auto-darkening helmet, leather gloves, copper backing bars, Cleco pliers, and a 4-1/2 in grinder with flap discs. That kit gets you through almost any panel replacement.
Preparing the Metal
Clean metal is non-negotiable. Grind or sand away paint, undercoating, and rust until you see shiny steel. On the back side too—rust hiding there blows holes in your weld. Remove the zinc coating 1/2 in back from the edge on galvanized panels. Wipe everything with acetone or brake cleaner right before welding.
Cut patches slightly oversized, then trim to fit with zero gap on butt joints or 1–1.5 in overlap on flanges. Clecos every 2–3 in keep things from moving while you tack. If the panel rocks even a little, the weld will crack later.
Choosing the Process
MIG – fastest for steel patch panels and rockers. Resistance spot welding – best for door skins and factory-style seams. TIG – aluminum hoods, stainless trim, or when you want perfect stacked-dime beads. Friction stir or laser – only if you’re joining aluminum to high-strength steel on late-model cars.
For most of us, MIG and spot welding cover everything.
MIG Welding Step by Step
Set the door chart as a starting point, then adjust on scrap the same thickness. Typical 0.8 mm (20 ga) settings: 17–18.5 volts, 50–80 amps, 250–350 in/min wire speed. You want a soft sizzling sound, not a sharp crack.
Push the gun at 10–15° angle, travel about 15 in per minute. Do 1/2–3/4 in stitches, skip an inch, let it cool, come back and fill the skips. Tack every inch first, then connect the tacks. Copper spoon or backing bar behind the joint prevents blow-through. On butt joints, leave 0.5–1 mm gap so the puddle has somewhere to go.
Example: replacing the lower door skin on a 1990s Silverado. Full-length seam, stitched every inch, skipped an inch, cooled with compressed air between passes. Final panel stayed dead flat, no filler needed.
Resistance Spot Welding in the Real World
Factory bodies use 4000–6000 spots per car. Home welders can’t match that speed, but 1–1.5 in spacing is plenty strong for repairs. Set timer to 10–14 cycles (60 Hz), force around 180–220 lb. Clean tips every 50 welds or the nuggets get weak. Test by prying two coupons apart—good spot leaves a button, not just a hole.
Example: new quarter panel on a ’69 Camaro. Overlapped factory flange, spotted every inch. Pulled over 1200 lb in shear before the steel tore.
Dealing with High-Strength and Galvanized Steel
Newer cars use boron and dual-phase steels that crack if you overheat them. Keep heat input low, use stitch pattern, and don’t quench with water. Galvanized needs the coating removed or you get porous welds and terrible fumes. Some guys run slightly higher wire speed and lower voltage to burn the zinc out as they go—works okay on light coating, still better to grind.
Common Problems and Fixes
Burn-through → drop voltage 1 volt or shorten stick-out. Warpage → more skip distance, clamp to a heavy table, weld from center out. Porosity → clean again, check for drafts blowing shield gas away. Cracks in weld → too much heat or contaminated metal.
Finishing the Welds
Grind with 40-grit flap disc to knock down high spots, then 80-grit to feather the edges. Don’t grind into the base metal more than 10 %. A quick pass with a DA and 120-grit leaves it ready for high-build primer. Planishing with a body hammer and dolly while the metal is still warm helps shrink stretched areas.
Conclusion
Welding car sheet metal well comes down to three things: clean metal, controlled heat, and tight fit-up. Do those every time and your patches disappear under paint instead of waving at you from across the parking lot. Start on scrap, dial in the machine, then move to the car. The first perfect butt joint you grind flush and can’t see in bare metal is addictive. Keep practicing, keep the heat low, and the car will look better than it did from the factory.
Frequently Asked Questions (FAQ)
Q1: Can a cheap 110 V MIG weld car body panels?
A: Yes, on 22–18 gauge with 0.023 wire and short stitches. Takes longer than a 220 V machine but works.
Q2: How far apart should stitch welds be on a rocker panel?
A: 1/2 in weld, 1–1.5 in skip is a safe pattern that keeps heat down.
Q3: Is it safe to weld over factory E-coat?
A: No—burns and makes weak, porous welds. Grind to bare steel first.
Q4: Do I need pure argon for steel MIG?
A: No, 75/25 argon/CO₂ gives better penetration and is cheaper.
Q5: How do I know if my spot welds are strong enough?
A: Pry two test pieces apart. You should pull a clean button out of one sheet.