Timber windows – stain failure: peeling/flaking
Causes
UV deterioration of the timber surface beneath the coating
Thermal expansion and contraction of the timber frame
Loss of adhesion due to excess moisture in the frame
Poor initial substrate preparation
The surface was damp or dirty when coated
The coating was fully transparent (which gives no UV protection to the timber)
Cause
UV deterioration of the timber surface beneath the coating
Repair
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film-forming coatings weather by peeling or flaking or becoming more opaque while penetrating stains weather by losing their intensity and general appearance.
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film-forming clear or lightly pigmented coatings are vulnerable to ageing of the underlying timber and loss of coating adhesion under UV light from the sun
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start the repair by removing all fittings and fastenings
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scrape or sand off the existing coating to give an as-new timber surface. Remove all dust and ensure the timber is dry before starting to stain or paint the windows
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recoating with the same type of product will restore appearance but frequent recoating will be required. Recoating of clear or pigmented film-forming stains is likely to be required every 6 months–2 years. Some timber producers do not recommend clear finishes for timber elements fully exposed to sun and weather
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if painting the windows, prime the sanded timber with timber primer
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fill holes and cracks with an exterior-grade timber filler and allow to harden, following the manufacturer’s instructions
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sand any filled areas
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prime sanded filler
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paint with a minimum of two coats of enamel paint starting with the edges of the sash. Waterborne enamels that have lower volatile organic compound (VOC) levels than oil-based paint, and are quicker drying, are available.
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major coating manufacturers have advice lines you can call for help and product recommendations for your specific circumstances.
Cause
Thermal expansion and contraction of the timber frame
Repair
- Repair as above. Use a lighter colour coating that will be less prone to absorbing heat.
Cause
Loss of adhesion due to excess moisture in the frame
Repair
- Repair as above. Ensure the frame is thoroughly dry before coating.
Cause
Poor initial substrate preparation
Repair
- Repair as above, ensuring that the existing coating is completely removed and the timber is clean and dry before coating.
Cause
The surface was damp or dirty when coated
Repair
- Repair as above, ensuring that the timber is clean and dry before coating.
Cause
A dark finish was used
Repair
- Repair as above. Use a lighter colour coating that will be less prone to absorbing heat.
Cause
The coating was fully transparent (which gives no UV protection to the timber)
Repair
- Repair as above. Some timber producers do not recommend clear finishes for timber elements fully exposed to sun and weather. Frequent recoating is likely to be necessary
Cause
Timber swelling/shrinkage caused by moisture absorption into timber end grain if joints were inadequately primed before installation
Repair
- Repair as above, ensuring that the unit is completely dry before coating.