Glazing putty for steel and metal windows: why timber putty fails on Crittall
Metal casement putty explained — why a non-absorbent frame needs a different oil blend, how to prepare rusted steel, and the sequence for re-puttying a steel casement without cracking the glass.
Steel casement windows — Crittall-type frames and their many imitators, ubiquitous in British houses built between the wars and now enthusiastically restored — are puttied, not beaded. They use a putty made specifically for them, and using the wrong tub is the reason so many re-glazed steel windows are still soft and greasy six months later.
Why timber putty does not work on steel
The distinction comes down to one word: absorbency.
Ordinary linseed oil putty — the “multi-purpose” tub — is formulated on the assumption that the frame will take up a small amount of oil from the putty’s back face. That slight oil loss is part of how the material begins to stiffen against the frame. It is also why a bare, unprimed timber rebate is disastrous: it takes far too much, and the putty starves.
Steel takes up nothing at all. Neither does a properly sealed dense hardwood. Multi-purpose putty against a non-absorbent surface cures fine at the exposed face — where the oxygen is — and can stay soft and oily where it meets the metal, sometimes for months. The fillet then slumps, refuses to hold a crisp arris, and lets go at the back where you cannot see it.
Metal casement putty is a different oil blend, formulated to set without help from the substrate. Data sheets describe it as intended for bedding and external fronting of single glass into primed steel frames, and it is equally the right answer for sealed hardwood and for glass set into stone or concrete surrounds. Its handling numbers are broadly those of timber putty: workable at +5 °C to +30 °C, about four hours of working time in the pot, and a skin in roughly two to four days outdoors depending on weather.
Preparation: rust is the whole job
On a steel window, glazing is the easy part. Preparation is where the time goes and where the outcome is decided.
Putty applied over rust lifts with the rust. Iron oxide occupies more volume than the steel it came from, so a rusting rebate is a rebate that is slowly jacking its own coatings off. Whatever you bed onto it goes with it, usually within a year.
The sequence:
- Remove the old putty and glass (see the method below).
- Get back to bright metal in the rebate. Wire brushing, mechanical abrasion or a needle gun, depending on how bad it is. On badly corroded frames this is genuinely laborious.
- Treat and prime immediately. Bare steel starts flash-rusting within hours in British humidity. Use a proper metal primer system — a zinc phosphate primer or an equivalent anti-corrosive — and do not let the frame sit overnight bare.
- Assess the frame honestly. Steel casements that have corroded through in the corners, or where the section has swollen enough to distort the opening light, need specialist repair or replacement of the affected member. That is beyond a putty job.
Original Crittall-type frames from the 1920s and 30s were not galvanised; the galvanised sections that resist corrosion so much better came later. If you are working on an interwar window, expect rust and budget for it.
Re-puttying a steel casement, step by step
The method mirrors a timber window, with four differences that matter.
- Soften and hack out. Warm the old putty with a heat gun on low or an infrared softener, keeping heat off the glass. Work the hacking knife down towards the frame. Old putty on steel is often even harder than on timber because there was no oil exchange with the frame to keep it plastic.
- Deal with the clips, not sprigs. Steel frames do not take sprigs — you cannot drive a pin into steel. They use spring glazing clips that locate into small holes or dimples pressed into the rebate. Keep the old clips if they are sound, note where they came from, and put them back in the same holes. Missing clips are available from steel-window specialists; improvising with a smear of extra putty is not a substitute, because the clips are the fixing.
- Prepare and prime the metal as above. This is the step that decides everything.
- Bed, glaze, clip and front as you would on timber: a continuous back bed of 1–2 mm, the glass pressed home at the edges, clips fitted, then the fillet pressed in and knifed to a bevel finishing about 2 mm below the sight line.
Then paint on the standard schedule: skin in 2–4 days, first coat at 7–21 days, final coat within 28 days, carried 1–2 mm onto the glass. The paint on a steel window is doing double duty — weatherproofing the putty and protecting the frame — so the repaint cycle is if anything more important than on timber.
Steel window quirks worth knowing
The rebates are shallow. Steel sections are slim, and the rebate depth is often only a few millimetres. There is less room for error, the fillet is smaller, and a narrower putty knife (25 mm rather than 50 mm) gives much better control.
The frames conduct heat and cold. A steel frame moves with temperature and runs with condensation in winter, which means the paint film at the putty-to-glass joint works harder than on timber. Inspect it annually.
Some interwar windows have leaded lights in steel frames. These are a different job again — the leaded panel is puttied or cemented into the steel rebate as a unit, and the panel’s own cames are bedded with leaded-light cement, not glazing putty.
Rust stains bleeding through new paint mean you did not get back to sound metal, or did not prime promptly. There is no cosmetic fix; it comes back.
Do not use putty with sealed units on steel frames
The prohibition is exactly the same as on timber, and the temptation is greater, because upgrading a cold single-glazed steel casement is such an obvious thing to want.
Linseed oil in any form — including metal casement putty — degrades the perimeter seal of an insulating glass unit, and the unit fogs. If you are fitting slim double-glazed units into a steel frame, use the bedding and fronting materials the unit manufacturer specifies, or an MS-polymer putty-replacement sealant tested as unit-compatible. Many steel-window refurbishment specialists now do exactly this, and their frames are re-formed with deeper rebates to take the units.
Aluminium and uPVC: not a putty job at all
For completeness, because the question comes up:
Aluminium windows are gasket-glazed. The glass is held by extruded rubber or neoprene gaskets in a channel designed for them. There is no putty in the system and nothing for putty to do.
uPVC windows are bead-glazed with sealed units, held by internal beads and glazing packers. Again, no putty — and any attempt to introduce it would sit against a sealed unit, with the usual consequence.
If your window is metal but the glass is held by a clipped-on bead rather than a knifed fillet, you are looking at a gasket or bead system, and the material you need is whatever that system specifies.
Quick summary
| Frame | Putty | Fixing | Note |
|---|---|---|---|
| Primed softwood | Linseed oil putty | Sprigs | Prime the rebate or it starves |
| Dense hardwood | Metal casement putty | Sprigs | Seal the rebate; too little absorption for timber putty |
| Primed steel casement | Metal casement putty | Spring clips | Rust removal decides the outcome |
| Steel with sealed units | Not putty | Clips or beads | Use the unit maker’s specified sealant |
| Aluminium | Not putty | Gaskets | Different system entirely |
| uPVC | Not putty | Beads | Different system entirely |
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