The durability of a painted timber window or door is decided before the topcoat goes on. Good preparation creates a clean, dry, stable surface and deals with moisture routes, failed joints, and bare timber first. Painting over dirt or a weak coating may look acceptable for a short time, but the defect will usually return.
First decide whether the substrate is ready
Check the timber, coating, glazing, seals, and hardware in daylight. Probe suspected soft areas carefully rather than sanding them flat. If the coating is very old or its composition is unknown, consider the possibility of lead and use an appropriate risk-controlled method. Our lead paint guide explains why dry sanding should not be the default on older joinery.
| Condition | Preparation response |
|---|---|
| Sound coating with minor dullness | Clean, lightly key, remove dust, and recoat with a compatible system. |
| Local chips or exposed timber | Feather stable edges, repair as required, and spot-prime bare areas. |
| Peeling or blistering | Remove failed material and diagnose moisture or compatibility problems. |
| Open joints or soft timber | Repair the substrate before painting; coating is not a structural repair. |
| Failed glazing or perimeter seals | Use the correct compatible system and preserve drainage details. |
The preparation sequence
- Choose a suitable weather window. Avoid rain, condensation, strong direct sun, and temperatures outside the coating specification.
- Protect adjacent surfaces. Mask glass carefully and protect seals, hinges, locks, drainage slots, and surrounding finishes.
- Wash and degrease. Use a method compatible with both the timber and coating. Allow it to dry fully.
- Remove failed coating. Work back to a firm edge without gouging the timber or damaging glazing.
- Complete repairs. Deal with open joints, minor defects, or local decay using a suitable joinery repair process.
- Sand for adhesion. Feather repaired edges and lightly key sound paint. See our sanding guide for abrasive selection and dust removal.
- Prime bare timber. Use the primer specified for the chosen coating system, including repaired areas and vulnerable end grain.
- Final clean and inspection. Remove dust and check that the opening still operates freely before applying finish coats.
Details that are easy to get wrong
- Painting flexible seals, brush piles, drainage slots, or moving hardware.
- Leaving sharp paint edges where failed coating meets sound coating.
- Using interior filler or an incompatible sealant externally.
- Closing the opening before the coating has cured sufficiently.
- Adding thick layers that bridge the sash and frame or change clearances.
Primer and topcoat compatibility
Treat primer, undercoat, and topcoat as one system. Mixing products without checking compatibility can affect adhesion, flexibility, and drying. Record the product, colour reference, batch, and application date so future maintenance is easier.
For a broader view of finish options and application stages, use our painting methods resource. If recurring coating failure is linked to water exposure, start with the timber weatherproofing checklist.
Frequently asked questions
Do you need to strip all existing paint?
No. Sound, compatible paint can often be cleaned and keyed. Remove material that is loose, poorly bonded, incompatible, or hiding a substrate problem.
Should glazing seals be painted?
Only if the specific glazing system and coating instructions permit it. Many seals and gaskets should remain unpainted.
Can you paint timber when it feels dry?
Surface feel alone is not enough. Recent rain, condensation, low temperature, or moisture within a joint can all affect coating performance.
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