How to Weatherproof Exterior Wood

To weatherproof exterior wood, you clean it thoroughly, let it dry completely, sand it to open the pores, then apply a finish that stops moisture reaching the timber fibres. You can use a penetrating oil that soaks in deep or a film-forming sealant that sits on top, applied in several thin coats. The prep work matters more than the product.

• A stiff brush and a good quality wood cleaner
• Household bleach for stubborn stains or black mildew spots
• Sandpaper, including medium grit around 80-grit for rough, weathered timber
• Clean water for rinsing
• A wood finish: wood paint, wood stain, wood oil, wood waterproofer or wood sealer

1. Clear the surface of grime, green algae and any flaky remnants of old, failing finishes using a stiff brush and a good quality wood cleaner.
2. For stubborn stains or black mildew spots, mix one part household bleach to three parts water, test it on a small hidden area first to check for discolouration, then apply.
3. Rinse the surface with plenty of clean water, removing every trace of cleaner or bleach, because any residue interferes with the sealant bonding to the wood.
4. Let the wood dry completely before going any further.
5. Sand the timber, starting with medium-grit paper around 80-grit on rough, weathered wood to knock down fuzzy, splintered fibres.
6. Avoid sanding with too fine a grit, as this polishes the wood, closes the pores and makes it harder for the sealant to soak in.
7. Choose your finish from wood paint, wood stain, wood oil, wood waterproofer or wood sealer, depending on whether you want to change the colour and how much protection you need.
8. Check how the product describes its ability to prevent water absorption, because terms like weatherproof and weather-resistant often mean the finish is not fully waterproof and will take up some moisture.
9. Check the product's breathability, since a low-breathability coating traps moisture inside the timber and can cause more problems than it solves.
10. Apply the finish in several thin coats rather than one thick one.

Wood is a natural, porous material, and its internal structure behaves like a tight bundle of microscopic straws ready to soak up any moisture they touch. When wood gets wet it swells, and as it dries it shrinks back. That cycle of expansion and contraction is what causes the damage.

Without a protective barrier, exterior timber warps and twists, cracks and splits, and eventually rots. Sustained dampness creates the ideal breeding ground for fungi, which break the wood down into a soft, crumbling mess. Damp timber also darkens, and patchy saturation leaves an uneven look. Separately, UV rays cause silvering, where wood becomes bleached and loses its natural pigment.

Water-resistant and waterproof are not the same thing. A water-resistant finish makes water bead on the surface for a while, but given enough time moisture finds its way through. A truly waterproof finish creates an impenetrable barrier that stops water reaching the wood fibres at all.

The type of timber matters. UK timber production is mostly softwoods at 94%, which vary in their natural ability to resist moisture. The remaining 6% is hardwoods such as oak, which are naturally denser and better at repelling water.

Moss, algae and lichens thrive in damp conditions. Their presence holds even more moisture against the wood, speeding up decay and giving the timber a weathered appearance.

If rot has already taken hold, it will only worsen until the source of the moisture is prevented. Even once moisture is reduced, rotted sections will likely need repairs to make the surface sound.

What actually causes damage to outdoor wood?

Wood absorbs large amounts of moisture, which leads to rot, organic growth and weathering. The swelling and shrinking as moisture levels change causes warping and splits, and those splits let in more moisture.

What is the difference between waterproof and water-resistant?

A water-resistant finish repels water for a time but moisture eventually gets through. A waterproof finish creates a barrier that stops water reaching the wood fibres in the first place.

Why does breathability matter?

Breathability is how easily water vapour passes through the wood. The higher it is, the more vapour breathes out naturally and the drier the timber stays. A low-breathability coating traps moisture inside.

Which finish should I choose?

Wood paint is opaque and covers the grain with a coloured finish. Wood stain keeps the grain visible while adding colour, often used on rough sawn timber such as fencing. Wood oil enhances the natural beauty of the wood with a hard, durable finish. A wood waterproofer is an invisible treatment that creates a water repellent finish while remaining highly breathable by chemically bonding to the timber. A wood sealer also gives an invisible water repellent finish, creating a protective film that seals the wood.