Painting Timber Window Frames: An Exterior Guide for London Homes

Why Timber Window Frames Need Regular Painting

Timber window frames are among the hardest-working components of any London building's exterior. They face rain, UV exposure, temperature fluctuations, and condensation — and they do so on narrow sections of wood with complex profiles, multiple moving parts, and junctions where water can pool and penetrate.

In London, where thousands of period properties retain their original or replacement timber sash windows, maintaining the paintwork on these frames is not merely cosmetic. Paint is the primary defence against moisture ingress into the timber. Once that defence fails — through cracking, peeling, or thinning of the paint film — water enters the wood, and rot follows. A repaint every five to eight years costs a fraction of what timber frame replacement or major rot repair involves.

Inspecting for Rot Before You Start

Before any painting begins, every frame needs a thorough inspection. Rot in timber windows is progressive and often concealed beneath apparently sound paint:

Visual inspection. Look for paint that is bubbling, flaking, or discoloured — particularly on horizontal surfaces like sills and the bottom rail of sash windows, where water sits. Dark staining on the paint surface can indicate moisture trapped beneath.

The probe test. Use a small screwdriver or awl to probe the timber at vulnerable points — sill ends, the bottom corners of lower sash rails, the junction between frame and sill, and any joints in the frame. Sound timber resists the probe firmly. Rotten timber allows the probe to sink in with little resistance.

Common rot locations in London sash windows. The most vulnerable points are the bottom rail of the lower sash (where condensation runs down the glass and pools), the outer sill (where rainwater collects), and the junction between the sill and the pulley stile (where water tracks into end grain).

If rot is found, it must be addressed before painting. Minor soft rot can be treated with a wood hardener (such as Ronseal Wet Rot Wood Hardener), which consolidates the softened fibres and provides a sound base for filling. Larger areas of rot require cutting out the decayed timber and splicing in new wood — a job for an experienced carpenter or joinery specialist.

Preparation: The Foundation of a Good Result

Preparation accounts for the majority of time spent on a professional window painting job, and it determines how long the finished result will last.

Scraping and sanding. Remove all loose, flaking, and poorly adhering paint using a scraper and sandpaper. The aim is to reach a sound, firmly adhered surface — not necessarily to strip back to bare wood everywhere, but to ensure that nothing loose remains. On London properties where multiple layers of old paint have built up over decades, a heat gun can be useful for softening thick paint films, but care is needed around glass (which can crack from heat) and putty (which becomes very soft).

Filling. Use a two-part epoxy wood filler (such as Repair Care or Ronseal High Performance Wood Filler) for any dents, cracks, or areas where rot has been treated. Epoxy fillers bond well to timber, can be shaped and sanded once cured, and are more durable than standard decorators' filler in exterior applications.

Replacing putty. Check the condition of the glazing putty on all panes. Cracked, loose, or missing putty allows water behind the glass and into the frame. Rake out defective putty and replace with linseed oil putty, leaving it to skin over for at least two weeks before painting. On listed buildings in conservation areas across Westminster, Chelsea, and Kensington, putty glazing rather than beading is often a planning requirement.

Sanding. Sand all prepared surfaces with 120-grit sandpaper to create a smooth, keyed surface for primer. Feather the edges where old paint meets bare wood to avoid visible steps in the finished surface.

Cleaning. Wipe all surfaces with a tack cloth or damp rag to remove sanding dust before priming.

Priming Bare and Repaired Areas

Any area of bare wood or filler must be primed before topcoating. The primer serves two critical functions: sealing the timber against moisture and providing adhesion for the topcoat.

Primer choice. For exterior timber in London, an aluminium wood primer (such as Dulux Trade Aluminium Wood Primer) is the traditional professional choice. It contains aluminium flake pigments that create a moisture barrier and reflect UV light, extending the life of the paint system. For a water-based alternative, Zinsser Bulls Eye 1-2-3 provides excellent adhesion and stain blocking.

Application. Apply primer by brush, working it into all joints, mouldings, and end grain. End grain absorbs moisture rapidly and needs thorough priming — apply two coats to sill ends and the bottom of sash rails.

Choosing the Right Topcoat

The topcoat must withstand London's weather whilst remaining flexible enough to accommodate the movement of timber as it absorbs and releases moisture seasonally.

Exterior gloss. Traditional oil-based gloss (such as Dulux Trade High Gloss) provides excellent durability and a hard, weather-resistant finish. Drying times are longer — typically 16 to 24 hours between coats — but the finished film is extremely tough.

Water-based exterior gloss. Modern water-based gloss formulations (such as Dulux Trade Quick Dry Gloss or Little Greene Intelligent Gloss) have improved significantly. They dry faster, yellow less over time, and have lower VOC emissions. However, they are slightly less durable than oil-based alternatives in exposed positions.

Exterior eggshell and satin. A satin or eggshell finish is increasingly popular for window frames, offering a more contemporary look than high gloss whilst still providing good moisture resistance. Teknos Futura Aqua 40 and Dulux Trade Diamond Satinwood are both suitable for exterior timber.

Flexible topcoats. Specialist flexible coatings such as Bedec Barn Paint or Teknos Aquatop are designed to stretch and contract with timber movement, reducing the cracking that leads to water ingress. These are particularly relevant for large sash windows on exposed elevations.

Application Technique

Brush choice. Use a 1.5-inch or 2-inch brush with fine bristles for glazing bars and mouldings, and a 2.5-inch brush for rails, stiles, and sills. Quality brushes — such as those from Hamilton or Purdy — hold more paint and lay off more smoothly than cheap alternatives.

Coat sequence. Apply one coat of undercoat followed by one or two coats of topcoat. On new or bare timber, the full system is primer, undercoat, and two topcoats — four coats in total.

Painting order for sash windows. If painting a traditional double-hung sash: reverse the sashes so you can reach the meeting rail and the bottom of the upper sash, paint those areas first, then return the sashes to their normal position and paint the accessible areas.

Avoiding paint on glass. Overlap the paint onto the glass by 1 to 2 millimetres — this seals the junction between putty and glass, preventing water from tracking behind the glazing compound.

Timing and Weather Considerations

London's weather makes exterior window painting a seasonal activity. The ideal conditions are:

• Dry weather with no rain forecast for 24 hours after application.
• Temperatures above 10 degrees Celsius.
• The surface should be dry and free of morning dew.

Spring (April to June) and early autumn (September to October) typically offer the best painting conditions in London. Midsummer can be problematic — direct sun on south- and west-facing windows causes paint to dry too quickly, leading to brush marks and poor flow.

We paint timber window frames across London year-round, scheduling exterior work around weather windows and prioritising the most exposed elevations first. If your windows are due for repainting, we are happy to inspect them and provide a detailed quotation.