Galvanised steel rusts at weld seams because the heat of welding burns off the zinc layer and leaves bare steel. The fix is to clean the weld area and apply a zinc-rich touch-up coating. For small touch-ups, Micron 2200 Zinc Aluminium Touch Up Paint is an aerosol whose page says it is for repairing weld seams, scratches and worn coatings. For large structures or site work by brush, Micron 2218 MetArmor Z is the brushable 5 L option.
Why it happens
- Welding heat burns off the galvanised zinc. The zinc coating on GI vaporises around the weld, so the bead and heat-affected zone are left as bare steel.
- The weld area is left unprotected. Without touch-up, that bare steel starts to rust while the rest of the structure stays protected.
You will see burnt zinc and rust around weld beads, and white or red rust on GI structures at the joints.
How to fix it
Choose by job size and how you want to apply it:
- Small touch-ups, aerosol convenience: Micron 2200 Zinc Aluminium Touch Up Paint. Its page describes a zinc-rich finish that forms a sacrificial zinc layer, lists galvanised steel and metal structures, and says it is for repairing weld seams, scratches and worn-out coatings.
- Large structures, site work, or brushing without spray equipment: Micron 2218 MetArmor Z Paint, 5 L. Its page describes a brushable zinc-rich coating for touch-up and repair on galvanised or bare ferrous metal, and lists repair of weld joints, scratches or cut edges.
Surface preparation matters more than the product. From the 2218 page:
- Clean the surface thoroughly to remove rust, grease, oil, dust and old paint.
- Use wire brushing, sandpaper or blasting to help the coating adhere.
- Make sure the surface is dry and free from contaminants.
- Stir 2218 thoroughly before and during use, and apply with a brush in even strokes.
The 2218 page gives drying times (touch dry, handle dry and full cure at 25°C and 50% RH) and the number of coats; follow those on the TDS. The 2200 page has no step-by-step directions, so follow the instructions on the can and the TDS, and prepare the surface the same way: remove slag, spatter, rust and grease, and let it dry.
What to avoid
- Do not apply over rust, grease or old paint. The 2218 page says the surface must be cleaned first, and the same applies to an aerosol touch-up.
- Do not touch up a weld that is still hot. Let it cool.
- Clean 2218 brushes and tools only with the solvents its page names (toluene, xylene or lacquer thinner), and keep those solvents away from sparks and flames.
- Do not expect a touch-up coating to restore lost steel. It protects what is there.
When to ask an expert
Hand off to Micron technical if:
- the work is structural, marine or specification-driven, for example a required zinc content or dry film thickness standard;
- you need the coating to match a client specification or a specific topcoat system.
If the steel under the rust has lost section, talk to a structural or corrosion engineer before coating.
FAQ
Why does galvanised steel rust after welding?
The welding heat burns off the zinc coating around the joint. That area is no longer galvanised and needs a zinc-rich touch-up.
Spray or brush: which should I use?
For small touch-ups, the 2200 aerosol is convenient. For large structures, many joints, or site work without spray equipment, the brushable 2218 in 5 L is the practical choice.
Do I need to grind the weld before touch-up?
Remove slag, spatter, rust, grease and loose material. The 2218 page suggests wire brushing, sandpaper or blasting to help the coating adhere.
Will the touch-up match the galvanised finish?
The 2200 page says it matches existing galvanised coatings. The 2218 page says it dries to a silver-grey metallic finish that visually matches galvanised surfaces.
See the product: Micron 2200 Zinc Aluminium Touch Up Paint. Related: 2218 MetArmor Z Paint. More welding fixes: welding hub.