How to Fill a Hole in Steel Plate Using a MIG Welder

Filling a hole in steel plate with a MIG welder is one of the more satisfying repair jobs you can tackle at home — but it does require good preparation, the right machine settings, and a steady hand. Done correctly, the repair can be as strong as the surrounding metal. Done poorly, you risk burn-through, porosity, or a weak weld that fails under load.

What You'll Need

Before you strike an arc, gather your equipment and consumables:

• MIG welder (a 130–180 amp machine suits most mild steel plate work)
• MIG wire — ER70S-6, 0.6 mm or 0.8 mm diameter is ideal for thinner plate
• Shielding gas — C25 (75% argon / 25% CO₂) is the most common all-purpose mix in NZ
• Angle grinder with a flap disc and a cutting disc
• Wire brush and angle grinder flap disc for cleaning
• Welding helmet (auto-darkening is strongly recommended)
• Welding gloves, leather apron, and long sleeves
• Clamps or magnets to hold a backer if needed
• Backing material — copper, aluminium, or a scrap of steel for larger holes

Step 1 — Assess the Hole Size

The approach you take depends on how big the hole is:

For very large holes, welding a patch is almost always the better structural choice. Trying to bridge a large gap with weld metal alone leads to excessive heat input, distortion, and poor fusion.

Step 2 — Prepare the Steel

Preparation is everything. MIG welding does not tolerate contamination well.

• Grind or wire-brush away all rust, paint, mill scale, and grease within at least 25 mm of the hole
• If the plate has been galvanised or zinc-coated, grind the coating back well beyond the weld zone — zinc fumes are toxic (see the WorkSafe NZ guidance on welding fumes)
• Clean the area with a rag dampened with acetone or panel wipe
• If the hole has ragged or torn edges, use a grinding disc to clean them up so the edge is smooth and consistent

Step 3 — Set Up Your MIG Welder

Correct settings prevent burn-through and porosity. As a starting guide for mild steel:

• Thin plate (1.5–3 mm): lower voltage (around 16–18 V), lower wire speed
• Medium plate (4–6 mm): mid-range voltage (18–21 V), moderate wire speed
• Thicker plate (6 mm+): higher voltage (21–24 V+), higher wire speed

Always run a test bead on a scrap piece of similar thickness before welding your repair. The weld should sound like a steady frying bacon sizzle — not a loud pop or a spluttery crackle.

Step 4 — Use a Backer for Holes Up to 25 mm

For holes in the 6–25 mm range, clamp or hold a copper or aluminium backer plate behind the hole. Weld metal won't fuse to copper or aluminium, so the backer gives you something to weld against without becoming part of the repair. Once the weld cools, the backer simply falls or pops away.

If access to the back is impossible, you can tack a thin scrap steel backer in place and weld over it — just know it will become a permanent part of the repair.

Step 5 — Fill the Hole

• Small holes: Hold the torch at roughly 10–15° from vertical, aim at the edge of the hole, and use a circular or weaving motion to build up weld metal gradually. Work in short bursts of 1–2 seconds, letting the puddle solidify slightly between passes to avoid burn-through.
• Larger holes with a backer: Tack the edges first at four points (north, south, east, west) to anchor the puddle, then fill in sections, always starting at the edge and working inward.
• Patch plates: Tack the patch at multiple points before running full seam welds. Weld in short, alternating runs (skip welding) to minimise heat distortion.

Allow the weld to cool naturally — never quench hot steel with water, as this can cause cracking.

Step 6 — Grind and Finish

Once cool, use a flap disc on your angle grinder to blend the weld flush with the surrounding plate. Check for porosity (small pits or holes in the weld bead) — if you find it, grind back and re-weld that section. Porosity indicates contamination or insufficient shielding gas coverage.

If the repair will be painted, prime with a weld-through or etch primer before applying a topcoat. Resene and Dulux both offer suitable metal primers through their NZ stockists.

Safety First

Welding carries real risks — take them seriously:

• Always weld in a well-ventilated area or use local exhaust ventilation
• Never weld galvanised, painted, or coated steel without grinding the coating back first — the fumes can cause serious illness
• Keep a fire extinguisher nearby and clear flammable materials from the work area
• Wear your PPE every time: auto-darkening helmet, leather gloves, natural-fibre clothing, and leather boots
• UV radiation from the arc can cause arc eye — never look at the arc without a proper welding lens

Refer to WorkSafe NZ for detailed guidance on welding hazards in the home workshop.

When to Call a Professional

Some welding jobs are best left to a qualified tradesperson:

• Structural steel — repairs to load-bearing members, trailers, vehicle chassis, or anything covered by the NZ Building Code may require certified welding to a recognised standard
• Pressure vessels and pipework — these are specialist areas with strict compliance requirements
• Thin or exotic materials — stainless steel, aluminium, and high-strength steels need different processes and skills
• If you're unsure of the steel grade — welding the wrong filler to certain alloy steels can create brittle, dangerous welds

If your repair is structural or safety-critical, find a qualified local welder on tradedirectory.co.nz who can assess the job and weld to the appropriate standard.

Sources & Further Reading

• WorkSafe NZ — Welding Hazards
• BRANZ — Steel and Metalwork Guidance
• MBIE Building Performance — NZ Building Code
• Resene — Metal Primers and Coatings
• Bunnings NZ — Welding Supplies
• Mitre 10 NZ — Welding Equipment