Which filler survives outdoors?

Rain, frost and daily movement destroy interior filler within a season — which products survive, and the preparation that decides whether any of them last.

Difficulty: beginner
Time: 25 min
Read: 4 min
Safety: caution

Short answer

Use an exterior-grade filler for shallow repairs and a two-part chemical filler for anything deep, loaded or in timber. Interior filler outdoors absorbs water, freezes and falls out within a winter. Whatever you use, deal with what caused the damage first and paint the repair, because almost no filler is ultraviolet stable on its own.

An outside repair is attacked in four ways that an inside one is not: rain soaks into it, frost expands whatever water got in, the substrate moves several millimetres a year with temperature, and ultraviolet degrades the surface. Interior filler is a porous, rigid, water-soluble-ish material and it loses on every count.

Safety

Much exterior filling is done from a ladder, and reaching sideways with a filling knife is how people fall. Move the ladder rather than stretching. Sanding paint on a home built before 1980 can release lead dust — wet-sand or leave it. Two-part fillers use a peroxide hardener that irritates skin and eyes; wear gloves.

Step by step

Find and fix the cause before filling anything.Filler is not a repair for a leak, for rot or for movement. A cracked cill that keeps opening, a rotten frame bottom, a render crack fed by a blocked gutter — the filler is the last step, not the first, and filling over an active cause guarantees the repair fails.
Cut back to sound material.Rake out loose render, cut out soft timber back to hard wood, wire-brush loose rust. Filler bonded to something that is already failing fails with it.
Treat timber before you fill it.Wood hardener into the remaining soft fibres, then a preservative, allowed to cure. Filling untreated decaying timber seals moisture in and speeds the decay up.
Choose the filler by depth and by what the repair has to do.Exterior-grade ready-mixed or powder filler for shallow work on masonry and render. Two-part chemical filler for timber, for depth, for anything that will be drilled or shaped, and for anywhere really exposed. Flexible exterior filler for a joint that moves — although a joint that moves properly wants a sealant, not a filler.
Do not use a sealant where a filler is needed, or the reverse.A static crack that will be painted wants filler, which sands flush and disappears. A joint between two materials that moves wants sealant, which stays flexible and cannot be sanded. Getting this the wrong way round is the commonest external repair mistake.
Build up in coats and leave the last proud.Thin coats dry and cure right through. Proud, then sanded flush, allows for shrinkage. Deep repairs done in one pass sink and craze.
Work in the right weather.Dry, above about five degrees, and with no rain or frost forecast for long enough to cure. Filler that freezes before it has set is destroyed. Check the working time and the recoat time, not just today's forecast.
Prime and paint the repair.Almost no exterior filler is ultraviolet stable, and an unpainted repair chalks and crumbles. The paint is part of the repair rather than a finish over it, and the repair needs primer because its absorption differs from the surrounding surface.
Check it after the first winter.The first freeze-thaw cycle is the real test. A hairline round the edge of a repair means water is getting behind it, and dealing with it then is far cheaper than in three years.

Common mistakes

Using interior filler outside because it was in the shed — It absorbs water, the water freezes, and the repair pushes itself out. It rarely lasts a winter.
Filling over rot — Decay continues under a waterproof cap, and the surrounding timber is worse when the repair eventually falls out.
Filling a moving joint — Filler is rigid. A joint that moves will crack it every year. That joint needs a low modulus sealant instead.
Leaving the repair unpainted over winter — Ultraviolet and rain break down an exposed filler surface. Prime it as soon as it is sanded.

Alternatives

Exterior-grade ready-mixed or powder filler: Formulated to resist water and frost, for shallow repairs to render, masonry and painted surfaces. Straightforward to use and must be painted.
Two-part chemical filler: Waterproof, dimensionally stable, sets in minutes and can be drilled and shaped. The right answer for timber repairs and anything deep or loaded.
Flexible exterior filler: A middle ground for repairs where slight movement is expected and the surface must be painted. Not a substitute for a proper sealant in a real movement joint.
Repair mortar: For masonry and render, a matched sand-and-cement or lime mortar is more durable, more breathable and matches the surrounding material better than any tub filler.
A timber splice: For a rotten cill or frame leg, cutting out and letting in a new piece of treated timber outlasts any amount of filler and is what a joiner would do.

Questions people ask

Can I use ordinary wall filler outside?

No. It absorbs water and is destroyed by frost, usually within the first winter. Exterior-grade or two-part filler is required.

What is the best filler for exterior wood?

A two-part chemical filler on timber that has been cut back to sound wood and treated with hardener and preservative. For anything structural or a large area, a let-in timber splice is better than filler.

Should I use filler or sealant on a crack outside?

Filler for a static crack that will be painted. Sealant for a joint between two materials, or a crack that keeps reopening. If it has cracked twice in the same place, it is moving and it needs sealant.

Do I have to paint exterior filler?

In practice yes. Very few are ultraviolet stable, and an unpainted repair chalks, absorbs water and fails. Prime it as well, because it absorbs paint differently from the surrounding surface.