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Why Preparation Matters More Than the Paint

A summerhouse sits in the toughest spot on the property: full sun on one elevation, driving rain on another, and shade that never quite dries out on the third. Paint on a shed or cabin takes far more punishment than paint on a house wall, because the timber moves, the joints flex and the weather never lets up. Most failed finishes you see peeling off garden buildings were not caused by cheap paint. They were caused by painting over damp, dirty or resinous timber and hoping for the best. Get the preparation right and a mid-range exterior finish will outlast a premium one applied in a hurry.

Choosing the Right Moment

Timber needs to be dry before anything touches it. That means a spell of two or three rain-free days, with more forecast, and moisture content low enough that the wood feels dry to the touch and looks pale rather than dark. Avoid painting in direct strong sun, because the surface heats up, the paint skins over too quickly and brush marks stay visible. Early morning or late afternoon in late spring and early summer is ideal.

• New timber: leave pressure-treated or tanalised buildings for a few weeks so any surface moisture can escape. Untreated new softwood needs only a light sanding before priming.
• Weathered timber: sand back to sound wood. Grey, silvered surfaces will not hold paint reliably.
• Previously painted timber: check whether the existing coating is holding firm. Flaking areas must be taken back to bare wood at the edges, then feathered smooth.
• Temperature: most exterior finishes want at least 10°C, and some water-based products need more. Read the tin rather than guessing.

Cleaning, Stripping and Sanding

Start by washing the whole structure down with warm water and a little mild detergent, working from the top downwards. Rinse thoroughly and let it dry. This removes the green algae and airborne grime that stops paint bonding, and it is especially important on north-facing walls that never see sun.

For old coatings that are blistering or flaking, a scraper and a heat gun will lift most of it, though a chemical stripper is kinder on the timber if you have a lot to remove. Once the surface is clear, sand with a medium grit, then finish with a finer grade along the grain. Pay attention to end grain on doors, windows and fascias, because this is where water gets in first.

Fill any cracks, nail holes and small splits with an exterior-grade wood filler, then sand flush. Do not fill large movement gaps between boards; those need to stay open so the timber can expand and contract without splitting the paint.

Treating Knots and Priming Bare Wood

Knots are the single most common cause of disappointing results on softwood summerhouses. Resin bleeds through paint as a brown stain, and no topcoat will hide it. Coat every knot and any resinous patch with a shellac-based knotting solution, and let it dry fully before priming. Two thin coats are better than one thick one.

Bare timber then needs a primer that suits it. For softwood, a solvent-based wood primer or an exterior wood primer is reliable; for hardwood, use an aluminium primer to block tannins. Prime all faces you can reach, including the underside of sills and the bottom edges of doors, because these are the areas that rot first. If you are using a modern exterior wood stain rather than paint, check the tin: many are self-priming on bare wood, but only if the timber is sound and clean.

Choosing a Flexible Exterior Finish

Summerhouse timber moves constantly through the year, so the finish needs to move with it. Rigid paints crack at the joints within a season or two. Look for products described as flexible, microporous or breathable, which let moisture vapour escape while keeping liquid water out.

• Exterior wood stain or woodshade: soaks in rather than sitting on the surface. Ideal for rough-sawn timber and the easiest to touch up. It needs recoating more often but never peels.
• Exterior gloss or satin: a classic look on smooth-planed timber. More protective but less forgiving, so preparation must be thorough.
• Water-based exterior systems: quick drying, low odour and easy to clean up. They have improved enormously and are fine for most garden buildings.
• Colour choice: pale colours fade and show dirt less than you might think, but dark colours absorb heat and can cause resin to weep. On a sunny site, mid-tones are the sensible compromise.

Application and the First Year

Stir the paint thoroughly; never shake it. Lay on a primer or first coat with a good-quality brush, working it into the grain and along any joints rather than just skimming the surface. Cut in around glazing and edges first, then roll or brush the flat panels. Two topcoats are the minimum, and three on the south and west elevations if the budget allows.

Mask glazing, hinges and handles before you start, or wipe paint off metal fittings immediately with a cloth dampened with the appropriate thinner. Keep a small pot of matching paint in the shed for touch-ups, because a summerhouse picks up knocks and scuffs at door edges and corners.

Finally, plan a maintenance coat every two to three years. A quick wash down, a light sand on any worn patches and a fresh coat takes an afternoon, and it is the single best way to keep the building looking sharp and the timber underneath dry for decades.