Mordant Solution (T‑Wash) for Galvanised Steel: Step‑by‑Step Painting Guide

A mordant solution (T-Wash) is a chemical pre-treatment designed for new, bright galvanised steel and similar zinc-coated surfaces. It reacts with the zinc to create a microscopic etch (a “key”), improving adhesion for the paint system you apply afterwards.

A practical benefit: you can see it working. When applied to a properly cleaned surface, the galvanising typically changes from bright silver to dark grey/black. This visible reaction confirms the surface has been chemically keyed and is ready for priming.

Use mordant solution when:

• You’re working with new/bright galvanised steel
• The steel is smooth and you can’t (or don’t want to) mechanically abrade it
• You want a visible confirmation that the surface has been chemically keyed

Be cautious or consider alternatives when:

• The galvanising is very old/weathered and reaction is patchy
• The surface is heavily contaminated (oils, silicone, traffic film) and cleaning is difficult
• You can sweep blast safely and have the equipment (often best for large industrial work)

Here’s the decision logic most painters actually need.

T-Wash mordant solution vs etching primer

• T-Wash: chemically etches the zinc before priming; reaction is visible
• Etching primer: a primer designed to bite into difficult substrates; faster workflow (no rinse step), but you don’t get the same visible “reaction check”

Rule of thumb: If you want maximum confidence on new galvanising, use T-Wash first, then prime. If you need a faster workflow and have a proven etch primer system, etch primer can be appropriate.

T-Wash vs sweep blasting

Sweep blasting (light abrasive blasting) creates an excellent mechanical key and is often preferred for large projects – if you can do it safely and correctly. T-Wash is typically chosen when blasting is impractical, too disruptive, or not permitted.

T-Wash vs weathering / “leave it outside”

Letting galvanising weather can help, but it’s unpredictable and slow. If you need a reliable, repeatable prep method on a project timeline, mordant solution is the more controlled approach.

Before you start, gather:

• Mordant solution (T-Wash)
• Degreaser/solvent appropriate for metal prep
• Clean rags (lint-free)
• Brush or clean applicator pad
• Abrasive pad (for stubborn areas)
• Clean water supply for rinsing
• PPE: chemical-resistant gloves, eye/face protection, long sleeves/trousers

Always wear appropriate PPE when handling mordant solution including chemical-resistant gloves and eye protection

STEP-BY-STEP GUIDE

Step 1: Inspect the galvanising (new, weathered, passivated?)

Look for:

• Bright, shiny zinc (typically new galvanising)
• White rust (zinc salts) or chalky residue
• Signs of passivation (often invisible, but shows up when T-Wash won’t react)

If there’s heavy corrosion, flaking coatings, or unknown contaminants, plan extra prep.

Step 2: Degrease properly (the step most people skip)

This is the biggest make-or-break step.

• Remove oils/grease thoroughly
• Change rags often
• If you suspect a stubborn factory film, use an abrasive pad after degreasing, then degrease again

Step 3: Apply T-Wash and confirm the reaction

Apply the T-Wash mordant solution liberally with a brush or clean rag.

What you want to see: A consistent change from silver to dark grey/black.

If some areas don’t change colour:

• Treat it as a contamination/passivation warning
• Re-clean/abrade those areas and re-apply until you get a consistent reaction

Step 4: Rinse thoroughly and dry

Once the reaction has occurred:

• Rinse thoroughly with clean water to remove residues/salts
• Allow the surface to dry completely

Don’t rush this. Residues left behind can compromise adhesion.

Step 5: Prime and topcoat within the overcoat window

After the surface is treated, rinsed, and dry:

• Apply a suitable metal primer or a compatible direct-to-metal (DTM) coating
• Aim to prime/topcoat within the recommended window (commonly within 48 hours after prep) to avoid surface contamination returning

T-Wash mordant solution applied to galvanised steel showing the characteristic dark grey reaction

Common mistakes that cause peeling on galvanised steel

• Painting new galvanising without a pre-treatment or suitable primer
• Skipping or rushing degreasing
• Not rinsing after mordant solution
• Leaving too long before priming (surface re-contaminates)
• Using an incompatible primer/topcoat system