How to repair a cracked metal trailer with a MIG welder

🔥 MIG trailer crack repair guide for Australian DIY users

What is involved in repairing a cracked metal trailer with a MIG welder?

Repairing a cracked metal trailer with a MIG welder can be a practical and cost-effective fix if the damage is limited to non-catastrophic cracks in the frame, drawbar attachments, side panels or tailgate sections. The key is to stop the crack from spreading, remove all fatigued metal, prepare a sound joint, and reweld using the correct MIG/GMAW settings, while checking whether reinforcement is needed.

For any trailer carrying road loads, suspension loads or tow coupling forces, repairs must be done carefully and should align with AS/NZS 1554 principles for structural steel welding. If the crack is in a highly stressed area such as the drawbar, spring hanger, axle mount or coupling area, professional inspection is strongly recommended before putting the trailer back on the road.

What skill level is required for MIG welding a cracked trailer?

Intermediate — Estimated time: 3 to 8 hours depending on crack size, access, rust condition and whether reinforcement plates are required

What tools are essential for MIG welding a cracked trailer in Australia?

What materials are needed for a durable MIG weld repair on a trailer?

Safety Requirements

Step-by-Step Guide

Step 1: Inspect the crack and decide if it is repairable

1️⃣ Assess the location

Look closely at where the crack is. Common repair areas on box trailers include side rails, tailgates, floor supports and mudguard brackets. Cracks in non-critical sheet or light structural sections are usually more DIY-friendly than cracks in the drawbar, suspension mounts or coupling area.

If the steel is heavily rusted, paper-thin, torn over a long distance, or bent out of shape, a simple weld-over repair will usually fail. In those cases, cut out and replace the damaged section rather than just filling the crack.

💡 If the crack sits at the end of a previous weld, that often indicates fatigue from poor design, no reinforcement, or too much flex in the trailer.

Step 2: Support the trailer and remove load from the damaged area

2️⃣ Stabilise the work

Park the trailer on level ground and chock the wheels. If the crack is in the chassis, frame or suspension-adjacent area, support the trailer securely on stands so the damaged section is not carrying weight during repair.

Remove any load, tools, cages or inserts from the trailer. If needed, jack and support the frame so the crack closes naturally before clamping.

Never weld a stressed crack while the steel is pulled open under load, as the repair may be misaligned or locked in tension.

Step 3: Clean back to bare steel

3️⃣ Remove paint, rust and contamination

Use a grinder with a flap disc or grinding disc to clean at least 20 to 30 mm either side of the crack to bright bare steel. Remove all paint, scale, rust, grease and old underbody coating.

If the trailer is galvanised, grind the coating well back from the weld zone and ensure strong ventilation. Zinc fumes are hazardous.

A clean weld zone is essential for stable MIG arc performance and proper penetration.

Step 4: Stop-drill the ends of the crack

4️⃣ Prevent crack spread

Use a drill bit, commonly 3 mm to 5 mm, to drill a hole at each visible end of the crack. This relieves the stress concentration and helps stop the crack from running further during welding or future use.

Take your time to identify the true end of the crack. Sometimes a hairline crack extends beyond what you first see, so use good light and a wire brush to trace it properly.

Step 5: Prepare the joint properly

5️⃣ Open the crack and remove weak metal

For steel around 2 mm to 3 mm, you may only need a light V-groove with a grinder. For thicker trailer frame steel around 3 mm to 5 mm, grind a clear V along the crack so the weld can penetrate into sound metal.

If the crack edges are ragged, thin or contaminated, cut the damaged section out and fit a fresh patch or replacement piece. Don’t just weld over split, rusted steel.

Aim to leave enough root at the bottom of the groove so the panel or section still holds alignment, then clamp it into position.

Step 6: Fit reinforcement if the area is fatigue-prone

6️⃣ Add strength where needed

If the crack formed in a high-flex area such as a frame rail, spring support bracket, side upright or tailgate hinge area, consider adding a reinforcement plate after the crack is repaired.

Use a plate of 3 mm to 6 mm mild steel shaped to spread the load beyond the cracked zone. Avoid square-ended patches where possible; use rounded corners or tapered ends to reduce stress risers.

Do not plate over an active crack without first fully repairing the crack underneath.

Step 7: Set up the MIG welder

7️⃣ Dial in the settings

For typical mild steel trailer repairs using 0.8 mm ER70S-6 wire with 75/25 argon/CO₂, good starting points are:

These are starting settings only. Fine-tune on scrap of the same thickness before welding the trailer.

Use the correct polarity for solid MIG wire with shielding gas: DCEP / electrode positive.

💡 Listen for a steady frying or buzzing sound. Harsh popping often means settings, stick-out or gas coverage need adjustment.

Step 8: Tack the repair and check alignment

8️⃣ Lock it in place first

Place small tack welds along the crack or around the fitted patch to hold everything in position. If using a reinforcement plate, tack it only after the main crack is properly repaired.

Check that the section is straight and that hinges, tailgates or frame members still line up correctly. This matters especially on tailgates and side rails where twist can affect latching.

Don’t fully weld until you’re confident the fit-up is right.

Step 9: Weld the crack in short controlled runs

9️⃣ Repair the main crack

For most trailer repairs, use short weld runs rather than one long continuous pass. This helps control heat input and reduces distortion.

On 2 mm to 3 mm steel, use a steady stringer bead with a slight push angle. On 3 mm to 5 mm frame material, weld into the V-prep so you achieve root fusion, then cap as needed.

If the crack is long, skip around the repair area:

This is especially important on thin trailer sides and tailgates.

If needed, grind and add a second pass where the groove was deep or the first pass sank below the surface.

Step 10: Weld on the reinforcement plate or fish plate

🔟 Strengthen the repair

Once the crack itself is fully welded and cleaned, fit the reinforcement plate over the repaired area if required. Clamp it tight with no gaps.

Weld the plate using stitch or continuous welds depending on the design and stress level. On lighter trailer sections, avoid excessive weld size that adds heat without benefit.

Make sure the reinforcement extends well past the original crack on both sides. A short patch placed directly over the crack only can create a new stress point at the weld ends.

💡 Tapered or radiused reinforcement plates last better than square patches with abrupt corners.

Step 11: Clean, inspect and test the repair

1️⃣1️⃣ Check weld quality

Clean slag-free MIG welds with a wire brush and inspect for:

If the repair is on a frame section, visually compare both sides for symmetry and check that no new distortion has been introduced.

For a DIY repair, a practical check is to lightly load the trailer and inspect for any visible movement, opening or cracking around the weld after initial use. Critical roadworthy items should still be assessed professionally if there is any doubt.

Step 12: Protect the steel from rust and return to service

1️⃣2️⃣ Finish properly

Grind only where needed for clearance or appearance. Do not grind structural welds flat unless necessary, as you can weaken the repair.

Clean the area, apply primer, then topcoat with a durable steel paint. Make sure any exposed internal seams or overlaps are coated as well as possible.

Reconnect wiring only after all hot work is complete. Before returning the trailer to service, check lights, guards, tailgate function, latch fitment and tyre clearance.

Common Mistakes to Avoid

Pro Tips

Recommended Tools