Why paint fails prematurely
Find out why paint fails and how you can fix the problem and prevent it altogether
By Roelof Strydom
“Paint is probably the most common exterior finish used on wood, metal and walls,” says Herman Rabe, technical service consultant at Prominent Paints. When paint is properly applied to the appropriate substrate, it can give a service life of up to 10 years. Herman explains that all too often, problems occur during the application of the paint and the paint coat fails to achieve the expected lifespan.
Why does paint fail prematurely?
Herman says, “There are two main factors contributing to early paint failure and we only have control over one of these.
The first is the weather and the second is application related.” The weather is the primary reason for paint failing and unfortunately this is completely out of our control. Herman explains that the sun’s ultraviolet rays cause paint pigments to fade. Radiation from the sun eventually breaks down the paint film, which leads to chalking.
The biggest enemy, however, is moisture and temperature changes which cause buildings to expand and contract. These movements stress the paint, weakening the bond between the paint and the substrate ,which leads to hairline cracks that quickly get bigger.
Moisture is a major problem, especially on exterior walls, as paint is porous and moisture can penetrate it. “It’s the most common cause of premature paint failure on wood, metal and plastered walls,” says Herman.
The paint on exterior walls is subject to wetting by various means such as rain and dew. Before painting it’s also important to clean areas that are protected from sunlight, like porches, eaves and walls covered by overhangs. Herman explains that dirty areas like these interfere with the adhesion of the new paint.
Paint also fails due to human error. This is when it’s applied incorrectly or the incorrect paint is applied; eventually leading to failure. “It comes down to the paint not adhering properly to the surface because the surface has not been treated, primed or prepared properly before applying the top coat of paint,” says Bennum van Jaarsveld, communications manager at Dulux.
Any raw surface requires a primer. Use plaster primer in the case of plaster or concrete, wood needs wood primer to seal and protect it, while metal requires a metal primer. If there are specific problems, like damp, these need to be addressed before any primer and topcoat can be applied.
Other problems include the application of too few coats, meaning the application is too thin, or when paint is applied over dirty or already peeling paint, which prevents the new paint from adhering properly.
A third problem relates to the choice of paint, such as using oil-based paint when acrylic should be used or vice versa. People also often choose cheap paint, for obvious reasons, but then pay the price later as cheap paints have inferior binders and pigments, leading to early failure.
Fixing the problem
* “Always prepare the surface properly by removing all the loose and flaky paint from the surface,” says Herman.
* Wash the surface with a solution of one part Progold General Purpose Cleaner to 10 parts water. Where fungus is present, wash the area with Progold Fungal Wash at a ratio of 1: 6 and allow for a reaction time of three hours. After three hours, scrub the area with a nylon brush or abrasive pad and finally rinse the surface with clean water.
* Allow the surface to dry properly, especially if it is a wood or cement surface before proceeding with any paint application.
* Make sure you are using the correct primer for the surface as not all primers are the same.
* In the case of rising damp or other moisture problems, use a reputable waterproofing company to first correct the waterproofing before starting with the painting.
* Always apply the primer and topcoats according to the product specification on the container. The product spreading rates are printed on the back of the container as a guideline and are there to guide you in applying the product to standard, ensuring the correct product performance.
* When painting parapet walls, apply two coats of Prominent Fibre Seal to the top of parapet walls before painting.
* Don’t stop at one coat of your selected topcoat. The second coat will provide a far superior paint performance and is required for the paint to perform as intended.
Preventing the problem
Bennum explains that you should rather spend a bit more time preparing and priming the surface before applying the topcoat to make sure it’s protected against moisture, dirt and rust.
Remember to adhere to the recoating time on the container. Herman explains that it is essential to waterproof parapet walls and house wall areas beneath the damp protective course. It is especially necessary on houses where the damp protective course is higher than the exterior ground level. He also notes that prevention is better than cure when it comes to the building a new house. It will save you a lot of money in the long run.
Troubleshooting paint problems
This is when a fine powder forms on the painted surface and will present itself when you wipe your hand over the surface. Possible causes may be the use of low grade and highly pigmented paint or using interior paint for exterior application.
The solution is to remove as much of the powder residue as possible using a stiff bristle brush or even a pressure washer on exterior surfaces. After the surface is clean and dry, apply a solvent- or water-based primer and then paint the surface with a topcoat.
These bubbles usually indicate a loss of adhesion in areas and the subsequent lifting of the paint film from the underlying surface. Possible causes may be that a solvent-based paint was applied over a damp or wet surface, moisture has penetrated the home through the exterior walls or the paint was exposed to high humidity and moisture shortly after it had dried.
If the blisters don’t extend all the way down, the solution is to remove the blisters by means of scraping and sanding and then repaint the surface with a water-based paint. If the blisters do extend all the way to the base material, you first need to locate and remove the moisture source. Next, remove the blisters by means of scraping and sanding, followed by an appropriate primer and then at least two coats of topcoat.
* Crocodiling or alligatoring
This refers to patterned cracks in the surface of the paint film that resemble the scales of a crocodile. The application of an extremely hard coating like a solvent-based enamel over a more flexible coating like a water-based primer could be a possible cause. Another reason for crocodiling could be that the topcoat was applied before the undercoat was dry.
To solve this problem, the old paint needs to be completely removed. You can use a heat gun to speed up the process. After the old paint has been removed, prime the surface with a water- or solvent-based primer. Once the primer has dried, apply two coats of water-based paint.
* Foaming or cratering
These are tiny bubbles that form when the paint is applied. As the paint dries, these bubbles then pop and leave small depressions, known as cratering. Shaking a partially filled tin of paint could be a possible cause as well as applying paint too rapidly, especially with a roller, or even applying a gloss or satin paint over a porous surface.
To prevent this from happening, buy good quality paint. All paint will foam to some degree, but good quality paints are formulated in such a way that the bubbles pop while the paint is still wet. Also avoid excessive rolling or brushing when applying the paint.
Premature or excessive lightening of paint can occur on surfaces that are regularly exposed to sunlight. Causes may be the use of paint that is vulnerable to UV radiation, such as bright reds, blues and yellows. The use of interior paint on an exterior surface can also lead to this problem.
Solve this problem by using quality exterior paint in colours that are recommended for exterior use.