How to Weld a Steel Mounting Plate onto a Hollow Post

Welding a steel mounting plate onto a hollow post is a common fabrication task for gates, fences, pergolas, and structural brackets around the home. Done correctly, it produces a strong, lasting joint — but hollow sections need a little extra care to avoid burn-through and distortion. Here's how to approach the job safely and get a solid result.

Safety First

Before you strike an arc, make sure your workspace and personal protective equipment (PPE) are sorted:

• Welding helmet with the correct shade lens (shade 10–13 for MIG/stick, depending on amperage)
• Leather welding gloves, long-sleeved cotton or leather jacket, and leather boots
• Adequate ventilation — weld outdoors or in a well-ventilated shed; steel fumes are hazardous
• Fire extinguisher nearby and flammable materials cleared away
• Welding screen or curtain to protect bystanders from arc flash

For full guidance on welding hazards, refer to WorkSafe NZ, which publishes health and safety information covering fume exposure, fire risk, and electrical safety in workshop environments.

Tools and Materials You'll Need

• MIG welder (most suitable for this job) or stick (SMAW) welder
• Angle grinder with grinding and flap discs
• Steel wire brush and rag
• Clamps or magnetic welding squares
• Measuring tape, combination square, and marker
• The steel mounting plate (cut to size and drilled if needed before welding)
• Matching filler wire or electrode for the steel grade

MIG welding (Gas Metal Arc Welding) is generally the easiest process for DIYers tackling thin-to-medium wall hollow sections, as it gives you good control over heat input. A 130–180 amp MIG welder running 0.8 mm ER70S-6 wire with C25 shielding gas (75% argon / 25% CO₂) suits most mild steel hollow sections and plates you'll encounter in NZ.

Step 1 — Prepare the Steel

Good prep is the single biggest factor in weld quality.

1. Clean all surfaces — grind or wire-brush any mill scale, rust, paint, or galvanising off the weld zone. Welding through coatings produces porosity and toxic fumes.
2. Check the fit-up — the plate should sit flush and flat against the post face. Gaps larger than about 1.5 mm will cause burn-through on thin wall sections.
3. Deburr edges — a quick pass with a flap disc removes sharp edges that can cause arc instability.

⚠️ Galvanised steel warning: Never weld galvanised (zinc-coated) steel without grinding the coating back at least 50 mm from the weld zone. Zinc fumes are highly toxic and can cause metal fume fever. If the post is galvanised, grind it back thoroughly and work in fresh air.

Step 2 — Position and Clamp

Accurate positioning before you weld saves a lot of grief:

• Use magnetic welding squares or clamps to hold the plate square and flush to the post
• Double-check alignment with a combination square
• Mark the plate position with a marker before clamping so you can recheck after tack welds

Step 3 — Tack Weld First

Don't run a full bead straight away. Place tack welds (short 10–15 mm welds) at opposite corners first, then check alignment again. Tacks lock the joint in position and let you make small adjustments before committing to the full weld.

Step 4 — Welding the Plate to the Hollow Post

This is where hollow sections need special attention:

Managing Heat on Hollow Sections

• Hollow sections have thinner walls than solid bar — typically 2–6 mm for standard RHS/SHS (rectangular/square hollow sections). Too much heat causes burn-through.
• Reduce amperage compared to what you'd use on solid stock of the same thickness as the plate.
• Use a stringer bead (no weaving) and keep travel speed consistent.
• Weld in short runs of 40–60 mm, then pause and allow the metal to cool slightly before continuing. This limits heat build-up and distortion.

Weld Sequence

Welding opposite sides in sequence minimises distortion caused by thermal contraction pulling the plate out of square.

Fillet Weld Size

For most mounting plate applications, a 6 mm fillet weld all around is a good starting point for mild steel. If the joint will carry significant load (e.g., a gate hinge plate or structural bracket), consult an engineer or qualified welder to confirm the required weld size.

Step 5 — Inspect and Finish

• Let the weld cool naturally — don't quench with water
• Wire brush the weld to remove slag (if stick welding) or spatter
• Inspect for porosity (small holes), undercut (groove along the weld toe), or incomplete fusion — these are defects that weaken the joint
• Grind smooth with a flap disc if a flush finish is needed
• Prime and paint exposed welds promptly to prevent rust, especially outdoors. Resene and Dulux both offer suitable metal primers and topcoats for NZ conditions.

Structural and Consent Considerations

If your mounting plate is part of a structural element — such as a post base for a deck, pergola, or retaining wall — the connection may need to comply with the NZ Building Code and potentially NZS 3604 (Timber-framed Buildings). Some structural welding work may require a building consent or sign-off by a suitably qualified engineer or other approved person. Always check with your local council before starting structural work. You can find more guidance at MBIE Building Performance.

BRANZ also publishes technical guidance on steel connections and building details that can help you understand what's required.

When to Call a Professional

Call a qualified welder or structural engineer if:

• The joint is load-bearing or structural (deck posts, carport columns, retaining structures)
• You're working with stainless steel, high-strength steel, or unknown steel grades
• The post or plate is galvanised and can't be fully de-coated
• You don't have welding experience or the right equipment
• A building consent or engineer's sign-off is required

A skilled tradesperson will ensure the weld meets the required standard and won't fail under load. You can find qualified local welders and fabricators on tradedirectory.co.nz.

Sources & Further Reading

• WorkSafe NZ — Welding Safety
• MBIE Building Performance — Building Code
• BRANZ — Technical Guidance
• Licensed Building Practitioners
• Resene — Metal Primers and Paints
• Standards NZ — NZS 3604 and Structural Standards