A clean sound surface is essential for the long-term success of any paint project. Even the highest quality paints will fail prematurely if applied to a surface that has not been prepared properly. There are some common obstacles that must be identified and properly addressed before painting can begin.
Mildew
Mildew is a fungal growth that can form on the surface of exterior and interior paint or directly on the surface of most substrates. It is usually black, gray or brown in color. It is most noticeable on white and light-colored surfaces. It can be visually confused with dirt. However, applying a few drops of household bleach will dissolve the discoloration if it is mildew.
Besides moisture, any or all of the following can influence the growth of mildew.
* Type of binder (latex resists mildew better than oil or alkyd binders)
* Porosity of the paint (glossy paints resist mildew better than flat paints)
* Film thickness of the paint (thick films resist mildew better than thin)
* Age of the paint film (newer paint resists better than weathered paint)
* Substrate (metal, especially aluminum and galvanized, resists mildew better than wood)
* Painting over mildew increases the chance that mildew will grow through the topcoat
* Shady areas, north-facing walls or under eaves foster mildew growth
If mildew is present, it can be removed with bleach. Light mildew can often be eliminated by simply adding a small amount of bleach to water. More serious growth can be removed by scrubbing the surface with a stronger bleach solution (one part bleach to three parts water).
Chalk
Chalk develops over a period of time, especially on old oil base paints exposed to sunlight. It appears as a white chalk powder on the surface of the paint film. As the paint weathers, the binder in the paint is degraded by sunshine and moisture, releasing the binders hold on the pigment.
In some parts of the country, a small degree of chalking is actually desirable because it tends to rid the surface of a certain amount of dirt and mildew. Too much chalking can harm a paint job in three ways:
* It can result in “run-down”, which can hurt the appearance of the surface below the painted area (such as a brick foundation),
* It can lighten the color of the paint
* It can erode the paint film
Chalk can be removed by scrubbing with a brush or broom, followed by a good rinsing with water. If chalking is severe, consider using a primer engineered to adhere to problem surfaces.
Staining
Stain bleed-through is a brownish or tan discoloration that can occur when paint is applied over certain types of bare wood. It occurs when tannins found in woods such as cedar, redwood and mahogany seep through the paint and discolor it. Staining can also be present over the knots of other wood species, especially pine.
To avoid staining, bare wood should be primed with a stain-resistant primer. Even though the primer itself may become stained, it will keep the wood tannins from bleeding into the topcoat.
Once staining has occurred, the discoloration should be removed from the exterior surface before applying a new coat of paint. The problem area should be sanded and then spot-primed with a stain-resistant primer.
Repairing the surface
Once the surface is clean, it must be made sound. The structure should be inspected for rotting and damage to the wood. Rotting and/or damaged wood should be replaced. Cracks, gaps and holes should be filled with a durable, paintable caulk, wood putty or any other appropriate filler. Old sealant that has cracked, eroded or pulled away from joints or cracks should be removed and replaced with new caulk.
Deteriorated or missing glazing compound should also be replaced.
Loose, flaking or peeling paint should be scraped and sanded before applying primers and topcoats. After scraping, feather the edges of any remaining paint to a smooth finish. Severe paint failures may require other methods of removal such as heat guns, chemical strippers, power washers and power sanders.
Final preparation
Thoroughly dust all the surfaces that will be primed or painted. Mask windows, doors and other surfaces not to be painted. Protect walkways, shrubs and flowerbeds with drop cloths. Remember, surface preparation will usually take twice as long to do as the application of the paint. But, it will be worth it.
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Author : Will Humbert
Company : Vista Paint
Will Humbert is an industrial coating specialist with Vista Paint.
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