Painting Window Sills in London: Interior and Exterior Techniques

Window Sills: Small Surface, Big Impact

Window sills take more punishment than almost any other painted surface in a London home. Exterior sills are exposed to rain, frost, UV and airborne pollution year-round. Interior sills bear the weight of plants, mugs and general clutter, and are cleaned repeatedly. Get the paint job wrong and you will be redoing it within a year. Get it right and a well-painted window sill will last five years or more.

This guide covers both interior and exterior sills, which require different approaches despite looking similar.

Exterior Window Sills: The Greater Challenge

London's climate is not especially harsh by northern European standards, but it is persistently damp. Exterior sills face standing water after rain, freeze-thaw cycles in winter and expansion and contraction with temperature changes throughout the year. Any weakness in the paint system — a crack, a poorly sealed putty line, a coat applied to damp wood — will be found and exploited.

Assessing the Existing Surface

Start by pressing a sharp implement into the wood at various points to check for rot. Soft, spongy areas indicate that water has penetrated the existing paint and the timber is deteriorating. Minor rot can be treated with a two-part epoxy wood repair system. More extensive decay will require the sill to be replaced — no amount of paint will fix structural rot.

Check the putty around the glass carefully. Old putty cracks, shrinks away from the glass and allows water to track down behind it. Any cracked or loose putty must be raked out and replaced before painting. Overpaint onto failed putty and you are simply sealing in the problem temporarily.

Preparation for Exterior Sills

• Remove all loose or flaking paint with a scraper and sand back to a firm edge
• Treat bare wood with a preservative wood primer, particularly the end grain which is most vulnerable to moisture ingress
• Re-bed any failed putty with linseed oil putty and allow it to skin over (at least two weeks) before painting over it
• Fill any cracks or holes with an exterior flexible filler
• Sand the entire surface smooth and clean off all dust

Paint Systems for Exterior Sills

For exterior timber sills, a traditional oil-based gloss system remains the most durable option. The paint film is harder, more water-resistant and more flexible than water-based equivalents. Apply a coat of alkyd primer, one undercoat and two topcoats, allowing each to dry fully before the next.

Water-based exterior paints have improved markedly but still struggle to match oil-based systems for longevity on exterior joinery, particularly on horizontal surfaces that pool water. If low-VOC paint is a priority, look at brands that specifically market water-based exterior trim paints with an extended open time.

Ensure the sill is completely dry before painting — no surface moisture and no rain forecast for at least 48 hours after the final coat.

Interior Window Sills: A More Forgiving Surface

Interior sills are considerably easier to prepare and paint than their exterior counterparts, but they still need to be done properly to last. In properties across Kensington, Mayfair and Chelsea, original timber sills are often in reasonable condition beneath layers of old paint and benefit from a careful refresh rather than a wholesale repaint.

Interior Preparation

For sound, previously painted interior sills:

• Wash with sugar soap to remove grease and grime
• Fill any chips or dents with a fine surface filler, sand smooth when dry
• Caulk any gap between the sill and the surrounding plaster or architrave with flexible decorator's caulk
• Lightly sand the entire surface to provide a key

For sills being stripped back to bare wood, prime with an acrylic wood primer and allow to dry fully before topcoating.

Paint Choice for Interior Sills

Interior window sills occupy a middle ground between walls and woodwork. They are cleaned regularly, potentially wet from condensation, and in direct sunlight in rooms that face south or west.

A water-based eggshell or satinwood finish is the most practical choice for interior sills in London homes. The finish is easy to clean, resists moisture reasonably well and will not yellow in sunlight as an oil-based white would. For a period property where a glossier finish suits the character of the house, an oil-based satinwood is a good option.

Avoid standard emulsion paint on window sills, even for interior use. It is not durable enough for the surface and will scuff and mark quickly.

Matching Interior and Exterior Colour

One consideration specific to window sills is the visual relationship between the exterior and interior finish. In Belgravia and Pimlico, where many properties have heritage listing constraints, the exterior colour may be fixed or require approval. The interior sill colour is generally free to differ, and many designers deliberately contrast the two — a white interior against a dark olive or deep green exterior, for instance.

Whatever colour is chosen, a consistent finish across all window sills in a room creates a polished, considered result. It is one of those details that professional decorators get right every time and is worth the care to do properly.