Marindec PaintsGuides / Application guide
Surface Examination
How to identify the substrate and assess contamination before painting.
Why surface preparation matters
Over 95% of coating failures are caused by poor surface preparation and application — not the paint itself. The most common root causes are:
- Contaminants not removed before painting (rust, millscale, oil, grease)
- Wrong primer selected for the substrate
- Coating applied below or above the target film thickness
Inspect the substrate before cleaning. Understand what you are working with.
Identify the substrate
| Classification | Substrate | Typical contaminants |
|---|---|---|
| Ferrous metals | Cast iron | Sand, mould release agents, iron oxide |
| Wrought iron / hot rolled mild steel | Iron oxide, millscale, magnetite | |
| Cold rolled mild steel (CRCA) | Tooling oils, grease, finger marks | |
| Stainless steel | Thin oxide layer | |
| Non-ferrous metals | Galvanised iron/steel | Grease, zinc hydroxide (white rust) |
| Aluminium | Colourless aluminium oxide | |
| Copper / brass | Surface oxides and compounds | |
| Cement-based | Plaster | Damp, dirt, loose deposits |
| Concrete / masonry | Dust, loose deposits, efflorescence (soluble salts) | |
| Fiber cement | Dust, dirt, efflorescence — alkaline porous surface | |
| Timber | Softwood | Resin at knots |
| Hardwood | Natural oils | |
| Previously painted | Any | Loose paint, soluble salts, rust, oils |
Assess contamination
Before cleaning, carry out a quick inspection:
1. Finger rub test — wipe the surface with a clean white cloth. Residue colour indicates the contaminant: black = millscale, orange/brown = rust, clear/oily = grease
2. Abrasive paper rub — rub lightly with fine abrasive paper and collect the particles on a clean cloth. Reveals the substrate's surface layer: millscale filings on hot-rolled steel, galvanised powder on zinc-coated surfaces, wood fibres on timber
3. Adhesive tape pull — press tape firmly to the surface, peel back, inspect particles on the backing
4. Penknife probe — probe or scrape lightly to check for loose scale or soft substrate beneath the surface
Water spread test
Apply a few drops of water to the untreated surface and observe how far they spread. This indicates the degree of surface contamination and guides coating selection.
| Rating | Initial contact angle | Final contact angle | Interpretation |
|---|---|---|---|
| 1 | 0° | 90° | Beads immediately — heavily contaminated, solvent-based coating required |
| 2 | 60° | 90° | Poor spread — solvent-based coating recommended |
| 3 | 30° | 60° | Moderate spread — solvent-based suitable |
| 4 | 0° | 30° | Good spread — water-based suitable |
| 5 | 0° | 0° | Full spread — water-based suitable |
Reference results for untreated substrates:
| Substrate | Water spread rating | Recommended preparation |
|---|---|---|
| Hot / cold rolled mild steel | 2 | Degreasing, rust & oxide removal, scrubbing |
| Aluminium | 2 | Degreasing, rust & oxide removal, scrubbing |
| Galvanised iron | 2 | Degreasing, rust & oxide removal, scrubbing |
| Plaster | 2 | Light to mild scrubbing |
| Softwood | 5 | Mild to tough scrubbing |
| Hardwood | 5 | Mild to tough scrubbing |
| Previously painted (sound) | 1–2 | Mild to tough scrubbing + spot prime |
Substrates rating 1–3 (metals) are suitable for solvent-based coatings.
Substrates rating 4–5 (wood, glass) are suitable for water-based coatings.
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