Oiling or varnishing decking: what you can do yourself

Oil or varnish: how to decide

Oil soaks into the wood fibres rather than sitting on top. It keeps the timber looking like timber, it is easy to patch in small areas without sanding back the whole deck, and it does not peel — when it wears out it just looks dry and grey again. The trade-off is that it needs redoing more often.

Varnish (or a decking lacquer) forms a film on the surface. It gives a harder, more polished look and holds up longer between coats, but that film can crack or peel once moisture gets underneath it, and a peeling varnish is a proper job to fix, usually a full sand back to bare wood.

For an open deck that takes rain, sun and foot traffic, oil is the more forgiving choice and the one we recommend to most people maintaining an existing deck. Varnish suits a covered terrace or a deck that stays drier, where the film has a better chance of staying intact.

Preparing the surface before either treatment

Whatever finish you choose, the deck needs to be clean and dry before anything goes on it. Sweep off debris, then wash with a stiff broom and a deck cleaner or diluted degreaser, working along the grain. A pressure washer works faster but keep the nozzle moving and at a shallow angle, otherwise it gouges soft spring-grown pine.

Let the boards dry fully before you touch them again — at least a day in good weather, longer if the timber is old and has soaked up water over the years. Check for and replace any rotten or split boards now, and re-drive or replace any screws that have lifted, since sanding over a raised screw head ruins the sandpaper and the board.

If the old finish is varnish that has started peeling, all of it needs to come off before a new coat goes on. Patching over flaking varnish just traps moisture underneath and speeds up the next round of peeling.

Sanding: how many passes and what grit

For a deck that only needs refreshing, one pass with 80-grit on a random orbital or belt sander is usually enough to open the grain and remove greying without taking off more wood than necessary. Sand along the length of the boards, never across, and keep the sander moving so it does not dish out a hollow in one spot.

If you are stripping old varnish or badly weathered oil back to bare timber, start coarser — 60-grit — then follow with 100-grit to smooth it before finishing. Skipping the finer grit leaves scratch marks that show up clearly once oil or varnish darkens the wood.

Vacuum or brush off all the dust afterwards, including from the gaps between boards, and wipe down with a slightly damp cloth. Dust left on the surface gets sealed into the finish and shows as a rough, gritty patch once it dries.

When the weather is right for the job

Both oil and varnish need dry, mild conditions to cure properly. Aim for a dry day above 15°C, with no rain forecast for at least 24 hours after application, and ideally not the hottest part of a summer afternoon, since the finish can skin over too fast in direct sun and trap solvent underneath.

Avoid working in high humidity or when dew is likely to settle overnight before the coat has dried — a common problem with an evening application in early autumn. Wind is less of an issue than sun or rain, though it will speed up drying enough that you should work in smaller sections.

Applying the finish and how long it takes to dry

Apply oil with a brush or roller, working along the grain, and wipe off any excess after 10 to 15 minutes with a cloth — oil that is left sitting on the surface stays tacky and attracts dust and leaves. Most oils need a second coat once the first has dried, usually the next day, and are fully cured for foot traffic within two to three days.

Varnish goes on thinner, in two or three coats, each left to dry fully before the next — check the tin, but a day between coats is typical. A light scuff with fine sandpaper between coats helps the next layer key in properly. Full cure before heavy use is usually a week or so, longer in cool or damp weather.

The mistake most people make is putting the deck back into use too soon. Furniture dragged across a finish that has skinned over but not cured underneath will mark it permanently, and you will not see the damage until the next time it rains.

• Clean and fully dry the deck before starting
• Sand along the grain, not across it
• Remove all dust before applying any finish
• Work in dry weather above 15°C, no rain for 24 hours after
• Wipe off excess oil after 10–15 minutes
• Let each varnish coat dry fully before the next
• Keep furniture and foot traffic off until fully cured

How often the treatment needs redoing

An oiled deck typically needs a fresh coat every one to two years, though a south-facing deck that gets full sun all day will fade and dry out faster than one in shade. You do not need to sand back each time — a clean, a light scuff, and a new coat over the old is usually enough.

Varnish tends to last two to four years before it needs attention, but when it does fail it fails as peeling rather than fading, and that means a full strip back to bare wood rather than a quick refresh. This is the main practical downside of varnish for anyone maintaining the deck themselves.

If the deck is large, awkwardly shaped, or the old finish has failed badly enough that stripping it is a full weekend's work, our decking and terrace service starts from 90 € per job and covers the sanding and refinishing. For a smaller deck or a single afternoon's help, odd jobs are billed from 39 € per hour.