How to clean and mechanically prepare each substrate type before priming or topcoating.

General rule

A substrate must be clean and mechanically sound before any primer or topcoat is applied. Painting onto dust or oil contamination causes the film to bond to the contaminant, not the substrate — resulting in adhesion failure regardless of the coating used.

Ferrous metals (mild steel, structural steel, iron)

Contaminants to remove: millscale, rust, iron oxide, oils, grease, dust

Select one or more methods depending on the degree of contamination and the environment of the finished coating:

Method	What it removes	Notes
Grit / abrasive blasting	Millscale, rust, oxides — highly effective	Creates anchor profile for adhesion; remove blast dust immediately before oxidation begins
Burn-off	Organics and oils (heat to ~400°C)	Leaves a residue — always follow with mechanical cleaning
Mechanical cleaning	Loose scale, particles	Wire brushing, grinding, vacuum or air-jet blowing, cloth wipe
Solvent cleaning	Oils, grease, organic contaminants	Use hydrocarbon solvent (xylene, lacquer thinners) or industrial solvent cleaner; wipe with a clean lint-free cloth; allow to dry fully

Recommended minimum for industrial/exterior environments: solvent clean + grit blast to Sa 2½ (near-white metal).

Recommended minimum for interior/standard environments: solvent clean + mechanical scrub.

Galvanised iron and zinc-coated steel

New galvanised surfaces:

Zinc coating produces white rust (zinc hydroxide carbonate) on exposure to air. This must be removed before painting:

1. Clean with galvanised iron cleaner or sugar soap solution

2. Rinse thoroughly with water

3. Allow to dry completely before priming

Weathered galvanised surfaces:

Lightly abrade the surface to remove the chalk layer that forms with age, then clean as above.

Do not use red oxide primer on galvanised steel — use GI Primer formulated for zinc-coated substrates.

Aluminium

Aluminium forms a colourless oxide layer naturally. This must be addressed before painting:

1. Solvent clean with a hydrocarbon solvent to remove oils

2. Lightly abrade with fine abrasive paper to break the oxide layer

3. Do not leave the surface exposed for extended periods between cleaning and priming

Do not use red oxide primer on aluminium — use an etch primer formulated for non-ferrous metals.

Concrete and masonry

Remove efflorescence (white powdery salt deposits) with a stiff brush; acid wash if heavily affected
Fill blow holes and surface cracks with an appropriate filler and allow to cure
Allow new concrete a minimum of 28 days to cure before painting
Ensure the surface is completely dry — test by taping a sheet of plastic to the surface and leaving for 24 hours; condensation indicates retained moisture

Previously painted surfaces

Before repainting any previously coated surface:

1. Remove loose, flaking, or peeling paint with a scraper or wire brush — spot areas only, not total stripping unless the existing system is wholly unsound

2. Feather back the edges of sound paint at the scrape boundaries with abrasive paper

3. Clean the entire surface with sugar soap or detergent solution

4. Rinse thoroughly and allow to dry completely

5. Apply primer to any bare metal or exposed substrate before topcoating

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Surface Preparation