Identify the surface and follow the current instructions for your exact products. Do not sand or strip uncertain old coatings. Read the safety guide before disturbing materials.
Stages one to four: inspection and stripping
Every good window job starts with a knife. Press the blade into the sill, the bottom rail of the lower sash and the meeting rails. Sound timber resists; rot gives. Rock each sash to see if it runs on its cords or if they are broken. Look at the putty: cracked, shrunk or missing putty is the most common way water gets in.
- Test for lead on pre 1970 paint and choose your method accordingly
- Remove the staff bead and lift out the lower sash if the job is more than a top coat, lay it flat on trestles
- Strip back failed paint to a sound edge, all the way to bare wood on sills and bottom rails
- Hack out any loose putty back to the rebate
Stages five to nine: repair
Rot is cut out, not filled over. Small areas take a two part wood filler or an epoxy repair system worked into sound timber. Larger areas are spliced with new timber by a joiner. Sills often need a whole new front section. The cords are replaced while the sash is out, because you will not want to take it out again for five years.
- Cut out soft timber to sound wood and let it dry, days if necessary
- Consolidate and fill with epoxy, shape when set
- Replace sash cords and check the weights balance the sash
- Ease the sash so it runs, and plane a whisker off if the new paint will bind
- Fill nail holes and open joints, sand everything to a smooth key
Stages ten to fourteen: priming and glazing
Bare timber is primed before putty goes anywhere near it, because putty stuck to bare wood loses its oil into the timber and fails. The putty is pressed in, tooled to a clean mitre at the corners and left to skin for at least a week before paint. This is where most rushed jobs go wrong.
- Knot any exposed resinous grain
- Prime all bare timber, including the rebates, with an oil primer
- Bed and face up new linseed putty, tooled to a crisp line
- Leave putty to skin: a week in summer, two in cool weather
- Undercoat every painted surface, with a second undercoat on sills and bottom rails
Stages fifteen to twenty: finishing
Gloss goes on in thin, full coats with an angled sash brush, cutting a line onto the glass by about two millimetres so the paint seals the putty edge. That overlap is not sloppiness, it is the seal that keeps water out of the joint. The sash is put back only when the paint is hard enough not to stick.
- First coat of oil gloss, laid off along the grain
- Sand lightly with fine paper and dust off
- Second coat of gloss, sealing two millimetres onto the glass
- Refit the sash and staff bead, check it runs freely
- Wax the runners with candle wax so the sashes glide
- Clean the glass with a blade and a dry cloth, and step back
Common mistakes
- Painting over cracked putty
- Putty on bare wood
- Gloss before the putty has skinned
- Painting the sash shut
- Skimping on the sill
If more than a third of a sash or sill is soft, or the case itself is rotten, you need a joiner before you need a painter. We can look and tell you honestly which it is.
Sources & further reading
Original general guidance from Aethos, informed by the resources below. These references cover relevant safety, materials or techniques, not a specification for your property. Product examples are not paid recommendations.
- HSE: lead in construction (opens in a new tab)
Why disturbing older lead paint requires a planned, controlled approach.
- Zinsser: AllCoat Exterior Satin (opens in a new tab)
A product example covering exterior substrates, preparation and application conditions, not a universal recommendation.
- Zinsser: Bulls Eye 1 2 3 technical information (opens in a new tab)
An example of why preparation, substrate suitability and recoat instructions are product specific.