How do I paint a fence or shed properly?

Preparation, weather and the coverage most people underestimate — how to make it last five years rather than two.

Difficulty: beginner
Time: 5 hr
Read: 2 min
Safety: caution

Short answer

Dry timber and dry weather matter more than the product. Check three or four dry days are forecast, brush off all loose material first, and work the coating into the end grain and joints — those are where failure starts.

Most fence and shed coatings fail early for the same reason: they were applied to damp wood, or to a dirty surface, or just before rain. The product does far less work than the conditions do.

Safety

Wear eye protection and gloves when spraying or brushing overhead, and a mask when sanding old paint. Check the weather — several dry days are needed. Keep preservatives away from ponds and plants you care about, and cover anything you do not want treated.

What you'll need

Wood treatment or paint
Stiff brush
Brush or sprayer
Sheeting
Sandpaper (optional)

Step by step

Check the forecast for several dry days.Dry before, during and after — most coatings need 24 hours or more to cure. Rain on fresh treatment mostly washes it into the soil.
Make sure the timber itself is dry.A fence can look dry and be wet inside after a wet week. Two or three dry days beforehand. Coating over damp wood traps moisture and causes flaking.
Brush off dirt, algae and loose material.A stiff brush and, if needed, a fungicidal wash. Coating over green algae means it grows through within a season.
Sand back any flaking coating.You do not need bare wood, but everything loose must go or the new coat lifts with it.
Cover what you do not want treated.Plants, paving, the lawn, the neighbour's side. Sheeting and a bit of care saves a lot of scrubbing.
Work the coating into joints and end grain.The end grain — the cut ends of boards and the tops of posts — absorbs many times more water than the faces. Failure starts there and it is the part most often rushed.
Do the bottom of the boards and behind, where you can reach.The bottom edge sits closest to damp ground and is skipped almost universally. It is where rot begins.
Apply a second coat if the product allows.Two thinner coats last considerably longer than one thick one, particularly on bare or previously untreated timber.

Tips

A sprayer is much faster for a long fence, but back-brush afterwards to work the coating in. Sprayed and unbrushed sits on the surface.
Buy more than you think. Rough sawn timber absorbs far more than the coverage figure on the tin suggests — often double.
New pressure-treated fencing should usually weather for several months before coating. Check the manufacturer's advice.

Common mistakes

Coating damp timber — The moisture is sealed in, and the coating flakes off within a season. Wait for a genuinely dry spell.
Skipping the end grain — It absorbs water many times faster than the faces. It is where rot starts, and it is the part most often missed.

Questions people ask

How often should I treat a fence?

Every two to three years for most products, and sooner on south-facing or exposed sections. Check the bottom of the boards and the post tops annually.

Can I paint a new fence straight away?

Usually not. Pressure-treated timber needs to weather for a few months first so the coating can penetrate. Check the manufacturer's guidance.