South African has a very unique climate; therefore there are certain paint problems that are specific to it. In, the next few articles we will address some of these problems and how to remedy them. In this article we will start with problems chalk and algae.
- Algae can grow on surfaces that are continually damp and dirty and receive no sunlight.
- Insufficient fungicide/ algaecide in the paint can also worsen the situation.
- Areas that have had previous mould, may have also been painted over.
- Reusing opened containers that have received contamination from application transfer and or storage conditions.
- failure to prime a bare wood surface before painting.
- to distinguish, firstly if the contaminant is fungus/algae or dirt, drop a few drops of household bleach onto the designated area and see if the discoloured areas disappears. if it does then it’s probably fungus/algae.
- check for any sources of moisture eg leaks, sealing or ventilation. Fix them.
- apply a surface sterilizer/fungicidal wash (a diluted household bleach solution – 1 part bleach>3 parts water can also be used) Leaving on for 20 minutes, applying more as it dries. Then scrub and rinse thoroughly. Remember to wear rubber gloves and eye protection while doing so.
- High pressure washing is a solution to fungus/algae (remember to rinse and residue and allow to dry again before applying any recommended paint)
- prime the surface with a water based primer.
- apply a premium exterior water based paint that is protected with a dry film biocide.
Chalking is the formation of a white, chalky powder on the surface of a weathered coating, it happens when the binder within the paint is broken down periodically due to weather conditions, UV rays and moisture exposure, the binder then releases the pigment articles within the paint, what you see as the white chalky power is consequently the broken down binder and released pigment particles. Nearly all paints will show some evidence of chalking over time, when they are subject to outdoor exposure. This slow erosion is much more preferable than cracking or flaking and once the chalk is removed the area is desirable again for painting.
You can identify chalking by rubbing the surface of the affected area with a damp cloth (or even dragging your hand) which results in a light deposit to the cloth and the restoration of the colour to the cleaned surface.
excessive chalking is not desired, because it can run down onto the underlying structure and deface the appearance of the surface. It can lighten the colour of your paint or erode your paint film resulting in a loss of protection to the substrate.
- using an interior paint quality for outdoors use ie enamel paints or lower gloss level acrylics.
- poor surface preparation and a failure to remove the poorly bound surface before painting – paint needs to adhere to a sound surface.
- Using a low grade paint comprising of a low binder and high pigment loading.
- use of an interior paint for an exterior application.
- not adequately sealing a porous surface.
- spreading a coat too thin.
- nearly all paints will show chalking over time as subjected to outdoor exposure – most south african medium quality paints are in the medium to high PVC range, and chalking can be expected within 3-4 years.
Chalk needs to be removed before repainting – it is is in the same category as dust or dirt.
- Rub the painted surface with a finger or dark cloth to determine the degree of chalking.
- high pressure cleaning is a common surface preparation method, and can be used to clean chalking.
- scrub with a stiff brush . Rinse thoroughly and allow to dry. Rub the surface with your finger to see if any chalk residue remains. Repeat the cleaning process if powder is still present. If chalk persists, recoat with Alkali Resistant/plaster Primer or Bonding Liquid and coat with appropriate quality top coat. If little or no chalk remains, and the old paint is in good condition, then no priming is necessary.
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