Painting Sash Windows in London: A Professional Trade Guide
How to paint sash windows correctly in London period homes — preparation, product selection, opening sequence, and the common mistakes that cause premature failure.
The Most Underestimated Job in Period Property Decoration
Sash windows are one of the defining features of London's housing stock — Georgian townhouses, Victorian terraces, Edwardian semis. They are also one of the most frequently botched decorating jobs. The common failure patterns are familiar to any experienced London decorator: sashes painted shut, drips on glass that were never removed, bare timber visible within a year of completion, or windows that stick and groan through seasonal movement.
Done properly, a painted sash window should last 8–12 years between full redecoration cycles, operate smoothly throughout, and look as sharp after a London winter as the day the brush was put down.
Understanding the Structure
A standard double-hung sash window has two independently moveable sashes — the upper (outer) and lower (inner) — running in channels within the frame. The sashes slide past each other, which means any paint that bridges the sash-to-channel interface will cause the window to stick. This is the primary mistake made by decorators who are unfamiliar with the mechanism.
There are also meeting rails (where the two sashes overlap at the centre), parting beads (the strips that separate the sash channels), and staff beads (the interior strips holding the lower sash in). Each of these surfaces needs attention but also needs to be painted in the correct sequence to avoid locking the mechanism.
Preparation
Strip back to bare wood if necessary. On older London properties, sash windows often carry 15–20 layers of paint. At this point, the moulding profiles are obscured, the sashes are heavy, and the window barely functions. Full stripping — hot-air gun, chemical stripper in recesses, careful hand scraping — is time-consuming but the correct answer. Painting over multiple thick coats simply accelerates the next failure.
Address rot. Probe all exposed sections with a sharp tool, particularly sill ends, bottom rails, and areas around glazing putty. Soft timber must be cut back to sound wood and filled with a two-part epoxy wood filler (Ronseal High Performance, Repair Care Dry Flex). Once cured and primed, these repairs are structurally sound and will outlast the surrounding timber. Do not use flexible fillers intended for gaps — they are not structural.
Reglaze where needed. Cracked or missing putty allows water in at the most vulnerable junction in the window. Hack out failed putty, prime the bare rebate, and reglaze with linseed oil putty (traditional and sympathetic to period frames) or acrylic glazing compound. Allow to skin before painting — typically 2–3 weeks for linseed putty, 24 hours for acrylic.
Sand all surfaces. 80-grit to remove raised grain and previous paint nibs, finishing with 120-grit for a smooth key. Clean down with a tack cloth before priming.
Priming
All bare timber, repairs, and previously stripped sections must receive a full wood primer before any undercoat or topcoat is applied. On older timber with a history of moisture ingress, an oil-based primer provides better penetration and moisture resistance than water-based alternatives.
Where knots are present, apply knotting solution first to prevent resin bleeding through subsequent coats.
The Painting Sequence
This is where the job is made or ruined. The correct sequence prevents painted-shut windows:
- Push the lower sash up and the upper sash down, so they are reversed from their normal positions.
- Paint the bottom rail of the upper sash and as much of the sides of the upper sash as are now accessible. Also paint the meeting rails of both sashes.
- Allow to dry (or proceed carefully with fast-drying products). Return the sashes to near-normal position but do not close fully.
- Paint the remainder of the upper sash, the lower sash in full, the frame reveals and soffit (top of the window opening).
- Leave sashes slightly open until fully cured.
Do not paint the parting bead or staff bead faces that contact the sashes — a light sanding and a wipe of linseed oil keeps these surfaces sliding freely.
Product Selection
Oil-based alkyd paints — traditional gloss — remain the professional preference for sash windows in London. They penetrate timber, provide excellent moisture resistance, and produce a hard, glass-smooth finish when applied correctly. The working time is longer than water-based equivalents, which aids levelling on detailed mouldings.
For clients who prefer a more contemporary finish: Little Greene Intelligent Eggshell (water-based) is the best performing water-based option, with good hardness and adhesion. Farrow & Ball's water-based eggshell works well where colour-matching to their interior emulsions is the priority, but requires careful application on large flat sections.
Do not use standard emulsion on sash windows under any circumstances. It is not durable enough for exterior exposure or the abrasion of daily operation.
Cutting Glass Lines
A sharp, steady cut against the glass — leaving a 1–2mm overlap of paint onto the glass — creates the seal that prevents water ingress at the putty junction. Use a small cutter's brush or a quality 25mm sash brush. Once dry, any paint on the glass beyond 2mm can be removed cleanly with a blade.
Professional Sash Window Decoration
Sash windows in London period properties are worth doing properly. If yours are sticking, failing, or long overdue for attention, contact us here to discuss the scope of work, or request a free quote and we will assess the condition and recommend the right approach.