The questions we get asked most, grouped so you can find yours. Longer explanations live in the complete guide.
Whiting — finely ground natural chalk — kneaded into linseed oil. That is the whole recipe for traditional putty. Chalk dominates by weight; the oil is the binder and the entire cure chemistry.
The oil blend. Multi-purpose putty part-relies on an absorbent frame drawing a little oil out of it to start curing at the back. Steel and dense hardwood absorb nothing, so metal casement putty is formulated to set without that help.
No, though many linseed putties are sold as usable for small filling jobs. Putty stays slightly flexible for decades and cures very slowly; a proper filler sets hard and sands. Do not fill a deep repair with putty and expect to sand it next morning.
Only cosmetically, and only if the paint ever wears thin. Both cure identically. Brown is a little more forgiving on dark-finished joinery.
No. Those systems are bead- or gasket-glazed and almost always hold sealed units. Putty has nothing to bond to and would damage the units.
Leaded-light cement, not glazing putty. It is a thinner, brushable compound worked under the leaves of the lead cames on both faces, then dusted back with whiting.
It skins in about two to four days in mild, dry weather. Full hardening through the fillet takes months, and in a sense never finishes — linseed putty cures by absorbing oxygen, not by drying out.
As soon as the skin will take a brush without wrinkling, usually 7 to 21 days. Data sheets ask for the final coat within 28 days. Later is not safer — unpainted putty crazes.
Ultraviolet light destroys the surface oil. The fillet crazes, then powders, then drops out — a single sunny British summer can be enough on a south elevation.
Yes, by 1 to 2 mm. That overlap seals the hairline joint between putty and glass, which is exactly where water gets in. It is the specified detail, not sloppiness.
Anything flexible and exterior-grade: a traditional oil system, a good flexible acrylic, or linseed oil paint, which is the historically matched partner and stays flexible for decades. Avoid hard, brittle films.
Between +5 °C and +30 °C, per the data sheets. Below about 5 °C the oxidation that cures putty effectively stalls, and a cold damp rebate will not bond. If you must glaze in winter, work on a dry mild day and accept a much longer wait before painting.
Yes, and this is the single most-skipped step. Bare softwood pulls the oil straight out of the putty, leaving a chalky ring that never cures and falls out within a couple of seasons. One coat of a solvent-borne wood primer is enough.
Squeeze the bed down to roughly 1–2 mm, evenly, all the way round. Continuity matters more than thickness — every gap is somewhere for water to sit.
Small headless pins driven into the rebate upstand that hold the glass. Yes, you need them: putty is not an adhesive. Fit them every 200–250 mm and at least two per side.
One confident, continuous stroke per side with the knife held at a shallow angle, one edge riding on the glass. Stopping mid-run and restarting is what produces a wavy line. Cut the corners last.
Either the putty is too oily — roll it on brown paper to draw the excess off — or the knife is dry. Wipe it on a rag or dip it in a little water between strokes.
Usually. Knead a fist-sized lump until body heat makes it plastic, and work in a few drops of raw linseed oil if it is still short and crumbly. If it has skinned right through, it is scrap.
No. The oil migrates into the sealed unit's edge seal and degrades it, and the unit fogs — often within a year or two. Sealed units are bedded on tape, silicone or an MS-polymer sealant instead.
Same answer. They are still sealed units. Use the bedding and fronting materials the unit manufacturer specifies, or a tested putty-replacement sealant.
Not directly. The oil attacks the plastic interlayer at the exposed edge and can cause hazing and delamination.
No. A bead-glazed rebate has neither the depth nor the geometry for a knifed fillet, and putty trapped behind a bead can neither cure nor be maintained. Bed the glass on the sealant intended for the system.
Yes — putty is designed for it. Cut back to sound material, prime any bare timber you expose, and knife a new section in. Whole-window stripping is rarely necessary.
Gentle heat. A heat gun on a low setting, kept moving, or an infrared softener. Keep heat off the glass, which cracks from thermal shock, and read the safety answer below before heating any pre-1990s paint.
Exclude air completely: press the surface flat, lay polythene or greaseproof paper directly onto it, seal the lid. For long storage a thin layer of water over the putty works well — pour it off and let the surface dry before kneading.
Treat it as if it may be. Putty made before the 1960s can contain white lead, some mid-century compounds contained asbestos fibre, and paint applied before 1992 in the UK may be lead-based. Never dry-sand or power-grind old putty or paint. Also: rags used with linseed oil can self-heat and ignite — lay them flat to dry outdoors or drop them in a lidded metal container of water.
