Why Preparation Determines Finish Performance
A coating's ability to bond to wood depends on the cleanliness of the surface, its moisture content, and the texture of the wood fibres. Grease, tannin bleed, previous coating residue, and biological growth (mould, mildew, algae) all create barriers between the new finish and the wood. Water in the wood fibres prevents oil-based products from penetrating and can cause water-based acrylics to blister as they dry.
Grey, weathered wood has a different surface chemistry than sound, exposed wood fibre. The grey layer is oxidised wood cellulose and dead fibre — coatings applied to it are bonding to a fragile surface layer rather than sound wood, which accelerates early failure.
Step 1: Structural Inspection
Before any cleaning, check the deck structure. Look for:
- Raised fasteners — nails and screws that have worked up from the boards. These should be set flush or slightly below the surface. Raised fasteners cause rust staining and potential injury.
- Soft or punky boards — probe suspicious spots with a screwdriver. If the tip penetrates more than a few millimetres without significant resistance, the board has internal rot and should be replaced before staining.
- Cupped or warped boards — significant cupping may require refastening. Applying stain to heavily cupped boards delays the need to address the problem but does not solve it.
- Cracked boards — surface checking is normal and can be addressed with preparation and coating. Deep splits that extend through the board warrant replacement.
Step 2: Cleaning
Removing Mould, Mildew, and Algae
Biological growth is common on shaded decks or on surfaces that retain moisture. Before applying any coating, organic growth must be killed and removed. The standard approach uses a diluted oxygen bleach solution (sodium percarbonate) or a dedicated deck cleaner containing mildewcide. Chlorine bleach is effective at killing surface organisms but can break down wood lignin with repeated use and may affect colour in some species.
Apply the cleaner to a dry or damp surface, allow it to dwell according to the product instructions (typically 10 to 20 minutes), then rinse thoroughly with a garden hose or low-pressure wash.
Pressure Washing
Pressure washing removes loose surface material, cleaning solution residue, and weathered fibre. The appropriate pressure for deck boards is generally in the range of 1,000 to 1,500 PSI with a fan tip. Higher pressure or a narrow tip can raise wood grain aggressively and damage softer boards. Always follow the wood grain direction when washing.
After pressure washing, allow the wood to dry fully before proceeding. In warm, dry weather this may take a day or two; in cool or humid conditions, it can take considerably longer. Moisture content should reach an acceptable level before staining — see Step 5 below.
Step 3: Wood Brightener (Where Applicable)
Weathered deck boards that have turned grey have an elevated pH at the surface from alkaline water exposure and biological processes. Wood brighteners — solutions containing oxalic acid or a similar mild acid — neutralise this pH change and open the wood grain. This step is particularly useful when:
- The wood is significantly greyed and needs restoration before staining
- Previous cleaning with bleach-based products has raised the surface pH
- The deck was pressure-treated recently (the treatment process leaves the surface with an elevated pH that can interfere with penetrating stains)
Apply the brightener to clean, damp wood, allow the appropriate dwell time, then rinse. The wood should visibly brighten in colour and the grain should appear more open. Allow to dry before proceeding.
Step 4: Sanding
Sanding is not always required, but it serves two functions: levelling raised grain after washing or weathering, and creating a slightly abraded surface that gives coatings better mechanical adhesion. Use 60 to 80 grit sandpaper for heavily weathered surfaces, and 80 to 100 grit for lightly weathered or previously coated wood that is being recoated with the same type of finish.
Sand with the grain to avoid cross-grain scratches, which become visible under semi-transparent stains. A pole sander covers horizontal surfaces efficiently. Hand sanding is sufficient for railings and trim.
After sanding, remove all dust with a dry brush or blower before staining. Dust mixed into a coating creates a rough, uneven surface and can affect adhesion.
Step 5: Moisture Check
Wood moisture content is the most common reason stain fails early. Oil-based penetrating stains require wood moisture content below a threshold that varies by manufacturer — often cited as 15% or below for most products. Water-based acrylics are generally more tolerant but still require the wood to be below full saturation.
A pin-type wood moisture meter is the accurate way to check this. Place the pins in the mid-face of a board away from edges or cracks. Take readings from several boards across the deck, as shaded areas dry more slowly. If readings are elevated, allow more drying time before proceeding.
The water-bead test is a simple field check: sprinkle a small amount of water on a prepared board. If it beads on the surface, the wood is not ready to absorb a coating. If it is absorbed within a few seconds, the wood is ready.
Step 6: Weather and Temperature Check
Most stains and sealers have application temperature requirements — commonly between 10°C and 32°C, with the surface out of direct sunlight. Applying stain to a sun-heated deck surface causes the product to dry too quickly before it can penetrate. Morning or evening application, or working in shaded conditions, gives better results in summer.
Check the forecast before starting: most products require at least 24 hours without rain after application. In Canada in spring and fall, this can mean waiting several days for a suitable window.
Working on Previously Stained Decks
The preparation approach depends on what type of finish is currently on the deck and how it is failing:
- Penetrating stain, no peeling: Clean, allow to dry, check moisture, and reapply. No stripping required.
- Film-forming stain or paint, peeling: The failing film must be stripped before reapplication. A deck stripper formulated to soften film-forming coatings, followed by pressure washing, removes most of the old coating. Residual coating in grain recesses is normal and acceptable.
- Unknown previous finish: Test a small area with a penetrating stain and observe whether it absorbs within the manufacturer's stated time. If it beads or takes much longer than expected, a film-forming residue is present and needs to be addressed before reapplication.