External paint coatings don’t last forever, and cleaning and recoating must be carried out routinely to maintain decorative and weather-resistant properties.
How long external paint will last depends on the:
- quality of surface preparation. Before paint is applied, the surface should be free of dust and grease and any gloss removed with light sanding
- quality of the paint
- level of exposure to the sun’s ultraviolet (UV) light. Paint on the south side of a building will last longer than that on the north or west side because it receives less UV light. (BRANZ found that for a test building in Wellington, the daily average UVA irradiation on the south wall was approximately 6 times lower than that on the north wall.)
- colour – lighter colours tend to last longer than darker because they absorb less heat, so expand and contract less
- size and type of material under the paint – paint on wide timber boards won’t last as long as on narrow boards because the overall movement in wide boards is greater, and paint on cement-based materials (concrete, cement plaster, fibre-cement products) tends to last longer than paint on timber
- cleanliness of the painted surface – wash down often to remove airborne chemicals and dirt from the paint
- number of coats applied
- underlying colour – applying a dark colour directly over a light one can cause a previously sound paint to lose adhesion because of the higher surface temperature.
Paint will last better when the old paint is still in a reasonably sound condition when recoated.
Interior repainting is usually motivated by the need to change the appearance rather than replacing a failing paint. However, interior paint or clear finish failure is more likely in areas with a lot of moisture, such as the bathroom or kitchen. Another fairly common problem is paint peeling off fibrous plaster surfaces. Moisture being absorbed into masonry internal walls can also cause paint failure.
The first signs of paint failure on timber are usually:
- at the corners of the board, because the coating is at its thinnest
- at joints or mitres, because moisture has been absorbed into the end grain of the timber, swelling it and causing the paint to fail.
Other common failure modes and causes are:
- blistering – caused by trapped moisture in the substrate, resin bleed in the timber or a heat source too close to the paint
- chalking – caused by UV weathering
- cracking – caused by recoating before the previous coat is dry, a hard coating applied over a soft elastic coating, paste and size left on the paint or movement in the material under the paint
- peeling, flaking – caused by an unsound surface, using the wrong paint for the substrate, using paint that is incompatible with the previous paint coating, leaving too long between coats, the surface is damp when painted, poor adhesion between coats, dark-coloured exterior paint applied over light or the paint coat is thinner where it is applied to the corner of the painted item.
To test how well a paint is adhering, make a number of criss-cross cuts (#) in it with a sharp blade, then press adhesive tape over them. If paint flakes come off with the tape, the paint should be removed. If the paint failure is localised, repairs can be carried out by sanding and spot priming before the final coating.
Updated: 20 July 2020