Rusting Steel Window Frames: Stop the Cause, Keep the Window
Owners assume steel windows rust from the outside in. In most of the 1920s–1950s frames we survey, the worst corrosion is on the inside face at the bottom rail — because that is where condensation sits for hours every winter morning.
The corrosion cycle
A steel section is a near-perfect thermal bridge; its internal surface runs close to outside temperature. Room moisture condenses on it and runs down to the bottom rail, where paint has usually already been broken by earlier flexing. Rust expands to several times the volume of the steel it replaces, which cracks the surrounding paint and lifts the putty line, letting in more water. Left alone the section eventually distorts enough to jam the casement.
Break the moisture supply and the cycle stops
Removing the condensation is more effective than any repainting cycle. A room-side secondary panel raises the temperature of the steel section by putting it outside the warm, humid room air, so the daily wetting simply stops. Combined with a proper wire-brush, phosphate and repaint on the existing corrosion, this is what keeps original frames in service.
- Treat existing rust back to bright metal first
- Zinc phosphate primer, then finish coats
- Secondary panel removes the ongoing moisture supply
Matching the look
Where the original is a black Crittall-type frame we powder coat the secondary profile to RAL 9005 matte so the two read as one assembly from inside. On galvanised or painted-white 1950s frames a satin white or a colour matched to the reveal usually disappears better.
Frequently asked questions
Rarely. Steel windows are repairable to a standard modern replacements cannot match, and specialist refurbishers routinely splice in new sections. Get the frame assessed before you write it off.
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South Yorkshire's specialists in secondary glazing — survey, measure, manufacture and install.