How to sand a deck and restore grey, weathered timber

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Your deck has come through winter silver-grey. The walkways feel rough underfoot, the old oil has gone patchy and you want to restore the timber before you re-oil. The question is how far to go: clean, sand, or strip back to bare wood. This guide covers how to read the weathering, when a clean is enough, how to sand a deck with a floor sander, and the prep each finish system asks for. When the deck is ready, our step-by-step guide to applying decking oil takes over.

Why a deck goes grey, and what else to look for

WoodSolutions notes that timber left unfinished in the weather fades to a silver grey and can distort, split and develop surface checking. Its deck design guide says those small checks come from intermittent wetting and drying and have no structural effect. Walk the deck and note what you find:

• Grey but smooth. A clean may be enough, depending on your finish.
• Rough or splintery. WoodSolutions says an unmaintained deck discolours and its surface becomes rough and prone to splinters. Plan to sand.
• Green growth or slippery patches. WoodSolutions adds that a poorly maintained deck is susceptible to mould, which can make the surface slippery. Plan a thorough clean.
• Black spots or dark run-off. See the stains section below.
• Flaking or patchy old coating. Plan to sand or strip it off.

Clean only, or sand? How to decide

Rubio Monocoat's data sheet says its Exterior Wood Cleaner removes grey discolouration, green deposits and dirt. On wood treated with another brand's saturating product more than a year ago, it removes all residues. If paint or varnish has been applied, the surface must be sanded, stripped or sand-blasted first.

On that last row, WoodSolutions warns that mixing different sealers or stains may stop the new coating adhering, and both Rubio exterior data sheets call for an old finish from another product to be removed completely.

Match the prep to your finish before you pick a grit

Decide on the finish before you hire the sander, because each system asks for a different surface and order of work.

Why not sand finer for a smoother deck? Rubio's data sheets say a smoother surface lets less of their oil and pigment in, so you get less colour and less protection. Two more checks before you commit:

• WOCA says softwood that has not been pressure-treated, including larch, must be primed with its Exterior Wood Primer. Red cedar is the exception.
• Rubio Monocoat Australia's DuroGrit warranty covers only durability class 1 and 2 timber, and Rubio advises the white and pastel DuroGrit colours for softwood only.

Sanding a deck with a floor sander

Rubio's Hybrid Wood Protector instructions for terraces sand off any old finish with screening nets, and allow the oil to be spread with a polishing machine and a 16 inch pad. For a belt sander, the sia 2800 zirconia belts we stock are 200 x 750 mm belts for 8 inch belt sanders, in grits from P24 to P100. Still choosing a machine? Our orbital vs drum sander comparison covers the trade-offs.

Before the sander goes on

1. Clear the deck, sweep it and get the debris out from between the boards.
2. Fix loose boards and any nails or screws that stick up. WoodSolutions' deck guide says nails should finish flush with the board, not punched below it, so moisture is not trapped.
3. Keep angle grinders and metal cutting away from the deck. WoodSolutions explains that iron filings left on the surface are likely to react with moisture and the timber's extractives and cause black staining.

Sanding the boards

1. Start with a grit coarse enough to cut through the grey layer and any old finish.
2. Step up through the grits, as in Rubio's 40, 80, 100 example above.
3. Stop at the grit your finish needs, from the table above.
4. Follow your finish's order of work. Rubio's DuroGrit data sheet says to clean with Exterior Wood Cleaner after sanding, while WOCA's order is clean, dry, then sand.

How to clean a weathered deck, step by step

Use the cleaner from the same brand as your finish. Rubio's exterior data sheets say to use only Rubio maintenance and cleaning products.

1. Dampen the deck. Rubio says to dampen the surface with a garden hose, and its Hybrid Wood Protector data sheet says never to use a high pressure cleaner.
2. Apply the cleaner. Rubio Exterior Wood Cleaner goes on undiluted. WOCA Exterior Wood Cleaner is mixed with water, at a stronger dose for very dirty wood, and left to work for five minutes.
3. Scrub along the boards. Rubio specifies a nylon scrubbing brush and WOCA a stiff-bristled brush. Work along the grain until the timber is clean, and repeat on stubborn stains.
4. Rinse. Rinse thoroughly until all the foam has gone.
5. Let it dry. Rubio allows about 30 minutes at 20°C with good ventilation before a Rubio finish. WOCA asks for at least 24 to 48 hours at 20°C before its Exterior Wood Oil.

Pick your weather and protect yourself. WOCA gives 10 to 25°C for its cleaner, ideally on a cloudy day. Both brands carry eye warnings for their cleaners, and Rubio says to wear gloves. Rubio's cleaner is also harmful to aquatic life, so take care near a fish pond.

Black marks and tannin stains

Check any marks that survive a clean:

• Black spots can be iron stain from metal filings, as described above.
• Stains on paving or brickwork below the deck can be tannin. WoodSolutions explains that most hardwoods contain water-soluble extractives that can leach to the surface whenever moisture leaves the timber.
• Tannin through a new finish. Rubio notes Hybrid Wood Protector is breathable and does not stop tannins leaching out when the wood gets wet.

If dark marks are still there once the deck has dried, call our technical line before you buy or apply a stain remover.

How to tell the deck is ready for oil

• Moisture. WOCA says to let the wood dry until its humidity is not above 17%. The DuroGrit data sheet gives a wood moisture range of 12 to 18%.
• Water test. The DuroGrit data sheet suggests sprinkling a few drops of water on the prepared wood. If they do not soak in within a few seconds, the wood is not dry or porous enough, so hold off.
• Weather. WOCA lists an application temperature of 15 to 30°C for its oil, in dry weather and out of direct sunlight. Hybrid Wood Protector should never be applied in direct sun or below 10°C. DuroGrit should not go on if rain is expected within 10 hours.
• Thirsty timber. The DuroGrit data sheet says consumption on very weathered or highly absorbent wood can, in exceptional cases, be twice as high, and adding up to 20% water can help control the spread rate.

From here, follow your product's data sheet and our decking oil guide for coats and cure times.

Not sure whether your deck needs a clean or a full sand? Call our sales team on 0403 964 703, or technical support on 0401 270 818 for application advice.

Sources

• Rubio Monocoat, Exterior Wood Cleaner, technical data sheet (PDF)
• Rubio Monocoat, DuroGrit, technical data sheet (PDF)
• Rubio Monocoat, Hybrid Wood Protector, technical data sheet (PDF)
• WOCA Denmark, Exterior Wood Cleaner, product page
• WOCA Denmark, Exterior Wood Oil, product page
• WoodSolutions, Decking, web page
• WoodSolutions, Technical Design Guide 21: Domestic Timber Deck Design (PDF)

Products in this guide

• Exterior Wood Cleaner - Rubio Monocoat prep cleaner for exterior timber
• Woca Exterior Wood Cleaner - WOCA prep cleaner for exterior timber
• 2800 200x750mm Zirconia Belt (8inch belt) - sia zirconia sanding belt for 8 inch belt sanders
• DuroGrit - 1L - Rubio Monocoat exterior wood oil
• WOCA Exterior Wood Oil - WOCA oil for exterior timber

All available from EcoGrit, shipped Australia-wide. Trade pricing available for flooring professionals.