Substrate Preparation Guide for Water-Based Timber Coatings

Before applying any water-based coating system, the substrate must be clean, dry, stable and properly sanded. Skipping preparation steps increases the risk of peeling, grain raising, discolouration and premature failure.

Moisture Content Requirements

• General timber: below 16% moisture content
• Accoya: below 8%
• Exterior hardwoods (e.g. oak): ideally 12–15%

Excess moisture prevents proper adhesion and can cause blistering, staining and early coating breakdown. Always confirm moisture content using a pin or pinless moisture meter before coating.

Step 1: Cleaning

• Remove dirt, dust, grease, mould and algae.
• Use mild, non-filming detergents.
• Rinse thoroughly and allow timber to dry fully.

Do not coat over contamination. Residues interfere with adhesion and can cause coating defects.

Step 2: Machining Quality

Raised grain and fibre movement often originate from poor machining rather than the coating itself.

• Ensure cutting knives are sharp.
• Use appropriate tooling for hardwood vs softwood.
• Replace blunt blades regularly to avoid surface damage.

Damaged or crushed fibres will swell when exposed to water-based systems.

Step 3: Sanding Timber Correctly

Correct sanding improves coating absorption and finish quality.

Recommended Grit Sequence

• Start: 80–100 grit (remove mill glaze)
• Finish: 120–180 grit for translucent systems
• Do not exceed 220 grit (can burnish the surface)

Avoid Swirl Marks

• Use a random orbital sander.
• Sand evenly across grain variations.
• Sand with the grain when hand sanding.
• Clean dust between grit changes.

Swirl marks and cross-grain scratches are highly visible under translucent coatings.

Preparing MDF for Water-Based Systems

MDF has a factory-pressed surface layer. This outer skin limits water absorption and must not be sanded.

Do Not Sand the Face

Sanding the surface destroys the pressed layer and increases swelling and fibre raising.

Edges & Profiles

• Lightly sand routed edges using P320–P400.
  
• Pre-spray edges 1–2 times with primer before full system application.

After drying, lightly de-nib edges by hand before applying the full primer + topcoat system.

Environmental Conditions During Coating

• Ideal temperature: 18–23°C
• Relative humidity: below 60%
• Avoid temperatures below 15°C
• Avoid high heat above 25°C with very low humidity

Low temperatures slow drying and reduce hardness. Excessive heat can cause poor levelling and air entrapment.

Repairs & Special Areas

• Fill cracks and holes with compatible filler.
• Allow filler to cure fully before sanding.
• Seal end grain and joints.
• Spot-prime knots or resinous areas where required.

How Proper Preparation Reduces Maintenance Costs

• Improves adhesion and coating lifespan.
• Reduces peeling and blistering.
• Improves finish quality.
• Reduces rework and material waste.

Preparation directly influences system performance. The coating can only perform as well as the surface beneath it.