By Benn Mumanyi
What causes exterior paints to fail? Actually all exterior paint that is exposed to the weather will eventually fail no matter how well it has been applied. This is due to cracking and the loss of adhesion or by erosion to the substrate. The period a paint lasts depends on a number of factors like the nature and condition of the substrate, type of coating applied and the severity of the of the weather conditions of the paint has to endure.
The above question should thus read, “What causes exterior paint to fail prematurely?”
Failure to smooth rough edges
Paint does not adhere well to an unstable surface such as old paint that exhibits marginal adhesion. Scraping is the usual method of removing this type of surface but it can require special attention because the paint that remains after the worst has been scrapped off will probably have rough edges.
When new paint is applied it flows over these rough surfaces edges and this result in inadequate thickness and creates areas of vulnerability where it can fail prematurely.
An effectively preventative measure is to taper the edges of the old paint by feather sanding them using medium grit garnet paper for general exterior use and finishing with fine grit garnet paper where close up appearance is important for semi gloss and gloss paint.
It is important not to be too aggressive during sanding because heat generated can degrade the adhesion of paint, which results in failures. If there is lead in paint it should never be sanded.
Failure to prepared weathered wood
Controlled exposure tests have clearly shown that a primer and paint does not adhere to wood that has been weathered as well as it adheres to wood that has not been weathered.
Even exposure for just a few weeks before painting can make a significant difference. The results after a year or two could be significant difference. The results after a year or two could be cracking and peeling rather than good performance from the paint for far longer.
Prior to priming or staining, the surface of weathered wood should be refreshed by thoroughly sanding. It is important to remove any wood that is grey from weathering. Medium grit garnet paper works well, but if a gloss or semi gloss paint is going to be applied, the wooden surface should be re-sanded with fine grit sand paper.
Failure to use a primer
Primers and sealers are coatings that enhance the appearance and performance of almost any paint job. Primers are applied because they improve the adhesion of the top coat of paint to the surface below, by providing a smooth, uniform surface, so that when paint is applied, it can get a better “grip” on the surface. This helps enhance the durability of the paint job.
Primers also help prevent stains and tannins coming through the paint and ruining its appearance. For example, top quality primers can block out stains caused by water, rust, smoke and other substances. The third reason for applying primers is purely aesthetic.
Failure to apply paint at the correct spread rate
If paint is applied too thin, many key properties are likely to suffer. These include crack resistance, mildew resistance, durability and in the case of primers, stain blocking and corrosion resistance.
The thicker the film of paint, the better the coating will perform in each of these properties. So be sure to follow the manufacturer’s recommended spread rate. Thinning the paint prior to application can also compromise these properties. Thinned paint applied at the recommended spread rate will dry to a thinner than intended film because of the reduced solids concentration and this affects performance.
Failure to use the correct type and quality of paint
The additional cost of quality paint goes a long way in the long run. The consumer will pay extra money for quality but it ensures more durability and better adhesion. The best quality paints are 100% pure acrylic paint. This investment will ensure long term cost saving as it means less frequent painting and a long lasting quality finish. It provides extra years of service and wards off some common problems that can render a paint job unsightly in a relatively short time period.
Consumers need to be aware of the description “super acrylic” as this term is misleading and does not refer to 100% acrylic products. To avoid disappointment, it is best to check with the experts on the exact constituents of the paint before purchase. Lastly it is important to choose paints where the manufacturer specifically details the application area and performance expectation of the paint.
The writer is the Technical Manager at Basco Paints
