External window paint must be kept in good condition to protect joinery and putty from the elements.
Sash and case windows must usually have their exterior repainted every five years. Repainting windows in good time, before paintwork has begun to break down, will minimise the preparation required.
Window sashes can usually be prepared and repainted in situ. If a more thorough job is required, sashes can be taken out fairly easily. The process of removing and replacing sashes is the same as when replacing sash cords.
You should select a suitable paint for outside use. You may need to apply for permissions for work involving a change in the colour of exterior window paint.
Follow manufacturers’ recommendations about:
- suitability and use of primers
- applying shellac knot solution to knots and resinous patches
- how many undercoats and finish coats are needed
When painting, ensure that the putty is completely covered and the glass to putty joint is sealed with paint. But avoid spreading the paint too far onto the glass surface and make sure you maintain a straight sight line.
Painting preparation
All surfaces must be prepared for repainting, taking care not to damage the wood. Bear in mind that the layers of paint are a record of the decorative history of the building. This would be entirely lost by stripping.
Where existing paint is in good condition:
- roughen the surface with sandpaper to help the new paint stick to it
- remove dirt and grease using sugar soap or washing up liquid in water
Loose and flaking paint should be removed either:
- using sandpaper
- with a thin bladed scraper
Sash lifts and other fittings can be removed if required. But leave the sash fastener in place if it’s in good condition – realigning this later is often tricky.
Paint stripping
Sometimes all or part of the existing paint must be stripped back to bare timber.
Paint stripping may be necessary where:
- windows have been badly neglected and paint has broken down
- paint has been poorly applied and the finish is rough and unsightly
- paint is thickly built up to the stage where mouldings are hidden
- the smooth running of the sashes is affected by multiple coats of paint
Caution should be exercised when stripping paint to avoid damage to timber, glass, putty and surrounding masonry. Protective measures may also be necessary as older paints may contain lead.
Alternative methods of paint removal:
- chemical paint strippers – aggressive caustic strippers are not recommended. More suitable chemical paint removers for hand application, e.g. dichloromethane and methanol solvent based products, are available
- mechanical sanding and scraping – care must be taken to avoid gouging the wood and eroding decorative details
- heat – using a blowtorch is not recommended. A hot air gun can be effective but a baffle must be used to protect the glass as it’s likely to crack even under this gentle heat
How to paint windows to prevent them sticking
- Pull the top sash right down, push the bottom sash up past it.
- Paint three sides of the top and bottom meeting rails, the lower half of the stiles and glazing bars, and the parts of the lower sash that you can reach.
- Paint the inner sill and the lowest 75mm only of the pulley stiles.
- Let the paint dry.
- Swap the position of the sashes, i.e. bottom sash right down and top sash halfway up.
- Paint the remaining parts of both sashes.
- Paint the top half of the pulley stiles.
- Do not paint the parts of the pulley stiles that are hidden by the sashes when they’re closed.
- Let the paint dry, then paint the surrounding woodwork.
