Painters Washington DC
Takoma Park, MD
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Why does paint fail? Many factors can affect the performance of a paint job, including the nature and condition of the substrate and the surface preparation; the conditions under which the paint was applied; how well the paint was applied; and of course the paint itself - its type and quality. Here are some things to take into account the next time you tackle a project.
First, essentially all exterior paint jobs will fail at some point, say by erosion through to the substrate or by cracking and loss of adhesion. So what we are really asking is: Why do paint jobs fail prematurely? Why is it cracking and peeling in only a year or two? Why is the paint blistering after the first rainfall? Why won't paint ever stick to this part of the wall? Why has the exterior enamel lost its gloss and color so soon? We demand a lot from today's paint. We want it to be easy to store and apply; to clean up with water if possible; to be easy enough for a first-time homeowner as well as a professional to apply effectively; to be available in any color we want; and of course, to be suitable for most surfaces, from wood to brick to stucco. A typical coat of paint is only about 1.5 mils (0.0015 inches) thick, and the protection of the surface provided by the paint depends on this thin film maintaining its integrity and continuity, its adhesion to the surfaces, and its resistance to the ravages of all sorts of weather conditions. Once the film splits, loses adhesion or begins to break down and slough off in the form of heavy "chalking," the protective or decorative service (or both) of the paint job is short lived. But if the paint is applied appropriately under suitable conditions to a properly prepared surface - assuming the paint is of suitable type and quality - the results can be excellent. Taking these points into account, and based on our experience at the Rohm and Haas Paint Quality Institute, here are seven aspects of a paint job that can lead to premature failure:
1. Surface preparation: Rough edges Painters generally know that paint will not tend to stick well to an unstable surface - one that has chalky pigment on it (as with old, weathered paint) or unbound sand (as with weathered stucco). They also know that removal of this type of material by scrubbing or power washing is important. But scraping off old paint with marginal adhesion can require special attention. The paint remaining after the worst is scraped off will often have rough edges. That's a problem, since the new paint can flow over the rough edge and end up with inadequate thickness right at the edge, creating a point of vulnerability where the new paint can fail prematurely. The preventive measure is to taper the edges of the old paint by "feather sanding" them, using medium grit (no. 120) garnet paper for general exterior use, and fine grit (no. 220) garnet paper where close-up appearance is important, as with semigloss and gloss paint. Be aware, though, that professional painters report failures related to aggressive feather sanding causing the old paint to loose adhesion. Power sanding, for example, can generate enough heat to degrade the adhesion of the old paint so that failures can result. (A note of warning here: sanding should not be done if lead may be in the old paint. If you suspect it is, call a contractor who can perform assessment and abatement.)
2. Substrate condition: Water from behind While you may have prepared the surface well, used top-quality paint and applied it properly; all bets are off if moisture gets behind the paint. Such moisture can result in blistering and lifting of the paint. Furthermore, on masonry surfaces, moisture from behind can carry white, crystalline salts out to the surface, which can undermine and lift the paint or accumulate on the paint and ruin the appearance. Water intrusion can also create those areas where paint never stays on. Typical ways in which water gets behind the paint are: .A crack or split in the wall material somewhere above that allows rainwater to enter.A leak or condensation from pipes inside the building .A faulty seal or caulking at corner joints or where siding meets trim, particularly window frames or headers .Cracked or open wall caps or chimney caps .An open chimney flue that allows rain to enter and run down the flue until it makes its way out into the wall material We see many paint failures on brick and stucco chimneys that could be avoided with an effective rain cap in place over the flue opening.
3. Paint application: Paint put on too thin While the painter may feel good about getting extra mileage out of each gallon of paint with acceptable appearance, many key properties may suffer if the paint application is too thin. After all, we are aiming to obtain a dry paint film that protects the surface being painted and provides a durable, pleasing appearance, and putting on a thinner film will obviously compromise this. Several factors impact how thick the film of primer or paint will be after drying. First, the spread rate will be important - that is, how many square feet each gallon covers. This is in turn impacted by how thick the liquid paint is and how heavily it tends to go on; how much the painter keeps spreading the paint out (some painters are experts at obtaining very high spread rates with almost any paint); and how rough and porous the surface is (its much easier to spread a paint too thin over a very smooth surface). The dry-film thickness after the paint dries is determined by the proportion of solids to liquid in the paint. The "solids" are the dry ingredients (mainly pigments and binder) that are left on the surface after the liquid portion (mainly water in latex paints and mainly paint thinner in oil-based paints) evaporates. This proportion is a characteristic of the particular paint formulation, and the formulator uses a volume relationship figure called the volume solids.
Generally speaking, higher-quality paints have higher volume solids than do economy paints, and so they dry to a thicker film for a given spread rate. Quality exterior latex paint typically has a volume solids figure in the 28- to 40-percent range. As you might have guessed, if one thins the paint before using it, the volume-solids figure is reduced, and to get the same dry film thickness as obtained before thinning, fewer square feet per gallon can be applied. It is important to strive to apply primers and paints at the spread rate recommended by the manufacturer, which takes into account the product volume solids and other factors.Generally, latex house and trim paints fall into the 350 to 450 square feet per gallon range when going over smooth surfaces, as in repainting wood or aluminum siding.
On the other hand, rough and porous surfaces, such as textured or weathered stucco, will require more like one gallon for 150 or 200 square feet. What performance properties are affected by dry paint film thickness? Here are several key primer and paint properties that depend very much on the thickness of the dry film: Crack resistance: Contractors who apply exterior elastomeric wall coatings (EWCs) know that they have to apply a very thick film if they are to obtain the bridging of cracks in a masonry or stucco surface, as is expected with this type of paint. The same is true with conventional primers and paints applied at conventional thickness over any material, especially wood. As the substrate expands and contracts with changes in temperature and humidity, the primer or paint on it must "absorb" these changes in dimension, or it will crack. Mildew resistance: The resistance to mildew growth on the surface of the exterior paint is related, in part, to the mildewcide in the paint.
Most mildewcides function as they are brought to the surface by rain and dew. The length of time that the paint job resists mildew then depends in part on how much mildewcide is available, which in turn is related to how much paint there is per square foot - leading back to spread rate. Erosion through to the substrate: Exterior paint fully exposed to the weather will be subject at some point to chalking. The binder in the film deteriorates to the point where pigment on the surface is released, and the paint "chalks." In this process, the paint erodes very gradually. If the film remains intact otherwise, this erosion determines the lifetime of the paint job, which is over once the substrate is exposed. All else being equal, including chalking and erosion rate, the life of the paint job will then be limited by the dry film thickness: a thicker film will take longer to erode than a thinner film. Primer performance: With primers, in addition to the impact of film thickness on cracking, mildew resistance and ultimate duration of erosion, two other key properties are affected: blocking of stains from the substrate and corrosion inhibition with ferrous metal substrates. These two important primer properties are directly impacted by dry film thickness.
4. Paint application: No primer used Using the right primer accomplishes the same benefits that you get from good surface preparation in general. The finish coat will display better adhesion, mildew and corrosion resistance, and uniformity of sheen or gloss and hiding. Surfaces not previously painted should be primed when painting, as well as areas exposed by loss of paint (e.g., peeled after years of exposure or removed as part of surface preparation). Truth be told, almost any paint job, even over existing surfaces that are sound and continuous, will benefit from application of an appropriate primer, as opposed to just applying two coats of paint ("self priming").
5. Surface preparation: Weathered wood not prepared Controlled exposure tests conducted at the Paint Quality Institute and others show that primer or paint will not adhere as well to wood that has been weathered as it will to the same wood that has not been weathered. Even exposure for just a few weeks before painting can make a significant difference. Prior to priming or staining, the surface of weathered wood should be refreshed by thorough sanding. Medium grit (no. 120) garnet paper works well. If a gloss or semigloss paint is ultimately to be applied, re-sand with fine grit (no. 220) sandpaper. Power washing can also be effective. But extreme caution must be used, lest the water jet cut into and damage the wood; for this reason it is best to not use power washing on softer siding woods such as cedar and redwood. Plain water without a cleaning agent or bleach should be used for power washing.
6. Paint application: Latex paint applied too cold People ask why latex paints have a minimum application temperature (typically 50° F) on the label or data sheet. The primary reason is that colder temperatures can keep the microscopic particles of binder in the paint from adequately fusing, or coalescing, and bind the pigment into a tough, continuous film. These particles are thermoplastic, meaning they harden as the temperature drops, and they cannot properly coalesce if the paint dries at too low a temperature. Remember, too, that the time needed for adequate film formation extends beyond dry-to-touch. We recommend not applying latex paint unless the temperature is predicted to not go below the minimum recommended application temperature for at least 36 hours.
Another factor to take into account is the temperature of the surface being painted; it must also be at or above the minimum application temperature at time of painting. What happens if the temperature is too low when the paint is applied? In extreme cases, the paint may crack as it dries, or even dry to a powdery consistency. More commonly, the appearance is fine, but film integrity and adhesion are poor, and what might have been a 15-year paint job lasts only two or three years. We saw one case where a quality exterior latex semigloss trim paint was applied to a garage door when the temperature was about 50° F, but it dropped to freezing that night. When we inspected the door several days later, it looked pretty good, but the applied paint would chip and shatter in front of your finger nail as you ran it across the paint. And what about too high a temperature? Applying latex paint under conditions that make it dry extremely fast may help in doing a second coat without waiting, but too fast a dry can also compromise film formation. The process of binder coalescence is time-dependent, so if the primer, stain or paint dries too quickly, the binder particles have inadequate mobility to form an optimum, durable film. Applying the coating under severe conditions that foster fast drying should be avoided. Try not to have more than one or two of these factors at the same time when applying the paint:
.High temperature: Ideally, apply paint when temperatures are below 90° F.
.Application in bright, direct sunshine: A dark paint can get quite hot as you apply it, through absorption of heat from the sunshine.
.Breezy conditions .Low humidity .Application to a dry, porous surface such as weathered stucco or cinder block that will tend to quickly absorb water from the wet paint 7.
Paint choice: Inadequate quality And of course there is always the quality - and suitability - of the paint used. A paint designed for the job is essential. For example, a gloss paint designed for use on metal will probably crack if applied to wood. And a thick elastomeric paint should not be used for a retaining wall that gets lots of moisture behind it. Rather, a thin coat of acrylic latex flat paint would be a good choice because it "breathes" well and will allow moisture vapor to pass through readily. And using an interior paint out of doors is asking for trouble. Going for low initial price with exterior paint can compromise protective properties, including adhesion; resistance to blistering, cracking, flaking and peeling; and resistance to alkalinity and efflorescence over masonry. Low price can also reduce color retention, mildew resistance, resistance to dirt collection and gloss retention.
Higher quality paints apply in a thicker film and generally have a higher volume of solids than do economy paints, and they provide a thicker dry film - one that is more uniform because it will tend to flow out better. They generally provide more effective protection against mildew growth on the surface of the paint and are made with pigments and binder that provide maximum durability. The Paint Quality Institute recommends using the top of the line product from your manufacturer or store of choice. Generally speaking, the best 100 percent acrylic latex paints provide the best overall performance. Compared to oil-based paints, assuming good surface preparation, these latex paints employing today's technology will provide far better long-term performance with respect to color and gloss retention, mildew resistance, and flexibility and crack resistance. But watch out for the areas of concern we've covered; nothing will take the place of proper surface preparation and application.