How to clean a wooden deck before sealing it

Sealing your decking is one of the best things you can do to protect it — but only if the surface underneath is properly clean. If you seal over dirt, algae, or old flaking product, you'll trap all of that in, and the new sealant won't bond properly. Knowing how to clean a wooden deck before sealing it is the most important step in the whole job, and it's one that's easy to rush or skip.

Here's how we do it, and how you can do it right too.

Why cleaning before sealing matters so much

Timber is porous. When you apply a deck oil, stain, or sealant, it soaks into the wood rather than sitting on top of it. If the surface is contaminated — with green algae, grey weathering, old product residue, or just general grime — the new sealant can't penetrate evenly. You end up with a patchy finish that peels within a season.

We've seen it plenty of times out in Nettleham and Saxilby — decking that looks freshly oiled but starts lifting at the edges within a few months, simply because the prep wasn't done properly. A clean, open-grained surface is what you're aiming for.

Step 1: Clear and inspect the deck

Before anything else, move all your furniture, plant pots, and anything else sitting on the boards. Check the deck over properly — look for:

• Loose or raised nails and screws — knock them back or replace them now
• Soft, spongy boards — these may be rotten and will need replacing before sealing
• Gaps packed with debris — leaves and muck trapped between boards hold moisture and speed up decay
• Any old sealant that's flaking — this needs to come off before you apply anything new

Don't skip this inspection. Sealing over a board that's already starting to rot just hides the problem.

Step 2: Apply a wood cleaner or deck wash

A dedicated wood cleaner does two things: it lifts ingrained dirt and kills off any algae, mould, or mildew that's taken hold. You can buy deck wash solutions from most builders' merchants.

Brush the solution into the boards along the grain using a stiff deck brush. Leave it to dwell for the time stated on the product — usually 10 to 20 minutes. Don't let it dry out on the surface; if it's a warm day, work in smaller sections.

For heavily greened or weathered decking, you may need to leave it a little longer or apply a second pass. You'll see the green and grey start to lift.

Step 3: Pressure wash carefully

Once the cleaner has done its work, rinse the deck off with a pressure washer. This is where people often go wrong — too much pressure, and you'll raise the grain and leave furry, splintered fibres across the boards.

Pressure settings for timber decking

Keep the pressure low — no more than 100–120 bar is a good guide for most softwoods like pine or treated timber. Hardwoods like oak or ipe can take a little more, but there's rarely any need to go higher.

Always wash with the grain, not across it, and keep the lance moving. Hold it at a consistent distance — around 20 to 30 cm from the surface. Never hold it static in one spot.

If you're not confident with the pressure washer settings, it's worth having a professional handle this part. Done wrong, it can do more harm than good — and reversing raised grain is a slow, labour-intensive process.

Cleaning between the boards

Use a flat surface cleaner or a narrow lance to clear out the gaps between boards. Debris trapped in there holds moisture and will undermine even the best sealant job.

Step 4: Let the deck dry fully

This is the step most people rush — and it's just as important as the cleaning itself. Timber needs to be completely dry before you seal it, or you'll trap moisture inside and the sealant will fail.

Allow at least 48 hours of dry weather before applying any sealant or oil. In Lincolnshire's changeable climate, that can mean watching the forecast carefully. Late spring and early summer tend to give the best windows for this kind of work.

If you're unsure whether the wood is dry enough, place a small piece of plastic sheeting flat on the deck and tape the edges down. Leave it for a couple of hours — if there's condensation underneath when you lift it, the deck needs more drying time.

Step 5: Light sanding if needed

If the pressure washing has raised the grain slightly — which can happen even with the correct technique — give the boards a light sand with 80-grit sandpaper along the grain before you seal. Wipe away the dust with a dry cloth, then you're ready to go.

For more advice on caring for timber surfaces, see our decking and wood cleaning guides.

When it makes sense to call in a professional

If your decking is heavily weathered, badly greened, or hasn't been cleaned in several years, DIY prep can be hard work and easy to get wrong. A professional pressure-washing service will have the right equipment, the correct cleaning products for your timber type, and the experience to avoid damaging the boards in the process.

We cover decking cleaning and restoration across Lincolnshire — from Welton and Cherry Willingham to Skellingthorpe and Branston. If you'd like to see what we offer before getting in touch, take a look at our full range of surface cleaning services, or check whether we cover your area.

Ready to get your decking properly prepped?

If you'd rather hand the cleaning over to someone who does this every day, we're happy to help. A well-prepped deck takes a sealant beautifully — and the results genuinely last.

Get a free quote today and we'll talk through what your decking needs.

Frequently asked questions

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Need a hand with yours? We cover Lincolnshire for patios, driveways, paths and more — see the areas we cover or browse our services. Get a free quote or call 07874 010739.