Treat a timber fence panel

Wait for the weather, properly

Timber needs a genuinely dry spell before and after, not just a dry afternoon. Two or three dry days beforehand let the surface give up the water it is holding, and a dry evening afterwards lets the finish take. A panel that feels dry to the hand can still be wet a few millimetres in, particularly on the shaded side.

Strong sun is as unhelpful as rain. It dries the surface of the finish before the timber has drawn any of it in, which leaves a film that sits on top and flakes. An overcast, still, dry day is the ideal, and the shaded side of a fence is best done in the morning.

Clean the panel down

Brush the whole panel with a stiff brush, working along the grain and getting into the gaps between boards. Cobwebs, leaf litter and the green film that builds up on a north-facing panel all stop a finish taking hold.

Scrape off anything that is flaking. There is no point in coating over a finish that has already given up; it will take the new coat with it when it goes. Feather the edges of sound material so the join does not show as a hard line.

Rinse if the panel is genuinely dirty, then go back to waiting. Washing a fence resets the drying clock, which is why the cleaning is done at the start of a dry spell rather than the end of one.

Repair before you coat

Check the fixings along each rail. A board that has worked loose will rattle in wind and will not sit flat under a brush. Re-fix it now rather than after the finish is on.

Look at the bottom of each board where it meets the ground or the gravel board. That is where water sits and where timber goes first, and it is worth knowing the state of it before spending an afternoon treating a panel that is close to the end of its life.

Keep the ground clear of the panel where you can. Soil and leaves banked against timber hold water against it permanently, and no finish outlasts that.

First coat, worked into the grain

Work from the top down and along the length of the boards, loading the brush generously and spreading it into the grain rather than across it. Timber drinks unevenly, and the end grain at the top and bottom of each board will take far more than the faces.

Push the finish into the gaps between boards and into the joints at the rails. Those are the places water gets in and the places a quick coat always misses.

Put something down to protect the ground. Treatment that lands on a paved area is difficult to shift once it has dried, and treatment that lands on planting is worth avoiding entirely.

Second coat, then leave it

Let the first coat take for as long as the container asks before the second. On thirsty timber the first coat can disappear almost completely, which is a sign that the wood needed it rather than a sign to put the second one on early.

The second coat goes on the same way and evens out the patches where the first was drawn in hardest. Two ordinary coats will always outlast one heavy one.

Then leave it alone through the next wet spell and look at it afterwards. The places where water beads are the places the finish took; the places where it soaks straight in are the places to start with next time.