Why preparation decides the finish

A shed takes a hammering that no other part of the garden has to endure. It sits in full sun, gets soaked in October, freezes in January and bakes again in June. Paint or stain is the only thing standing between that timber and a slow descent into grey, cracked boards. The frustrating truth is that most failed shed coatings weren't let down by the product – they were let down by what happened before the tin was opened.

Budget a weekend for a standard 8ft by 6ft shed. Two-thirds of that time will go on cleaning, sanding and repairs, and only a third on actually applying the coating. It feels unbalanced, but a well-prepped shed will hold its finish for five to eight years rather than two.

Choosing between paint and stain

Both protect timber, but they behave very differently, so pick before you buy anything else.

• Paint sits on the surface as a film. It hides grain and colour, gives you a huge choice of shades, and is easier to wash down. The trade-off is that it can crack and flake, and once it starts, you'll be scraping and sanding before you can recoat.
• Stain soaks into the wood and colours it rather than coating it. It never peels, it lets the timber breathe, and touching up a scuff is a thirty-second job. It won't hide imperfections, and heavily weathered boards will always show their history through it.
• Water-based products dry quickly, clean up with soap and water, and have far less smell. Solvent-based options often penetrate better on older, thirstier timber and cope with damp conditions during application.

Whichever you choose, make sure the tin says it's for exterior timber. Interior paint or stain will fail within a season outdoors.

Cleaning the timber properly

Dirt, algae and old flaking coating all stop a new finish from gripping. Work around the shed from the top down.

• Brush the whole structure with a stiff hand brush to lift cobwebs, dust and loose debris, paying attention to the underside of the eaves and the corners where dirt collects.
• Wash with warm water and a dedicated wood cleaner or a mild detergent, using a soft brush. Rinse thoroughly – residues left behind will interfere with adhesion.
• Treat green algae, black mould spots or mildew with a suitable fungicidal wash, then let the timber dry completely. This usually means two or three dry days.
• Take a look at how the timber behaves. Sprinkle a few drops of water on a board: if they bead up, there's old coating or sealer still present. If they soak straight in, the wood is bare and ready.

Don't pressure-wash a shed. It drives water into joints, splinters the surface and can force moisture behind the cladding where it will do real damage.

Sanding, filling and repairs

Once the shed is dry, deal with everything the coating would otherwise highlight.

• Sand rough patches, raised grain and any grey, weathered surface with a medium-grade abrasive, then go over the whole thing lightly with a finer grade. Always sand along the grain.
• Scrape back any loose or flaking paint to a firm edge, feathering the boundary so you don't leave a ridge. Fill the resulting hollow with an exterior-grade wood filler, then sand flush.
• Treat bare knots with a knotting solution. Resin will bleed through most finishes and leave a brown stain that no amount of extra coats will hide.
• Replace rotten beading or damaged boards now. Coating timber that's already decaying just seals the problem in.
• Brush or vacuum off all the dust, then wipe down with a slightly damp cloth. Dust is the single most common reason a finish feels gritty.

Applying thin coats

Check the forecast and the label. Most exterior finishes want temperatures above 10°C, dry timber and no rain for at least 24 hours after application. Avoid painting in direct sunlight or in a breeze – both make the coating dry too fast to level properly.

Mask window frames, hinges, handles and anything you'd rather didn't get coated, then work from the top of the shed downwards so drips land on untreated timber. Start with the roof edge and fascia, then the walls, then the door and its frame.

Stir the tin thoroughly rather than shaking it, and decant what you need into a smaller container so the bulk stays clean.

• Apply the first coat thinly, working the brush into the grain and into the gaps between boards. Thin coats penetrate and cure properly; thick ones skin over, stay soft underneath and fail early.
• Allow the full drying time stated on the tin – typically four to six hours for water-based products, longer for solvent-based. Cool, damp weather stretches that considerably.
• Lightly sand between coats if the surface feels at all rough, then wipe off the dust.
• Add a second coat, and a third on south-facing walls, end grain and anywhere that catches the worst of the weather. End grain soaks up finish like a sponge, so give it extra attention.

A quality 50mm brush for flat panels and a smaller angled brush for edges and beading will see you right. If you're spraying, mask far more generously than you think necessary and keep the nozzle moving to avoid runs.

Looking after the finish

Once it's done, inspect the shed twice a year – ideally in spring and early autumn. Look for scuffs near the door, hairline cracks on the sunniest wall and any spot where water sits. Cleaning off dirt and leaf litter from the base and behind the shed does as much for the timber as the coating itself.

Touch up small areas as soon as you spot them, while they're still small. A ten-minute repair once a year will save you a full weekend of scraping and sanding further down the line, and it keeps a modest garden shed looking cared for rather than neglected.