PAINT APPLIED TO NEW CEMENT PLASTER OR CONCRETE
A developer, builder, or owner of a building may elect to have paint applied to concrete walls (tilt-up or cast-in-place), or to portland cement plaster, in order to color the surface in a particular decoration. Painting is suitable procedure if proper preparation of the surface has been performed, good quality-primer and paint have been selected, and correct application procedures for each of the several coats have been followed. I sometimes encounter deficiencies in paint that has been applied to concrete or portland cement plaster. Typically, in those cases, the project has suffered from improper application practices. This article will discuss some matters related to painting concrete and plaster. Normally, I have observed that paint problems develop on relatively new concrete or plaster, rather than an older base.
Paint must not be applied to new cement plaster too soon after the plaster has been applied to a wall, or one or more of several possible problems may develop. Also, paint should not be applied to concrete too soon after the concrete has been formed into a wall.
Paint must not be applied until cement plaster has been adequately moist-cured, to gain desired strength and toughness, and then allowed to lose most of the excess mix water by evaporation from the surface. Moisture within the mass of concrete or plaster also must have time and favorable conditions to allow excess moisture within the wall to migrate outward to the surface and evaporate. Under no circumstances should paint be permitted to trap excess moisture within the mass of a concrete wall or cement plaster membrane. If paint is brushed, rolled or sprayed onto new plaster before proper curing, followed by drying, have occurred, delamination of paint may be expected. As the paint delaminates or peels from the surface of plaster, it may even peel some grains of sand, and some cement, from the surface of the mortar. When cleaving of paint from the base occurs, the deficiency is a painting problem, not a plastering problem. Note that I employed the word "painting" to indicate a problem with application of paint, not the paint product per se.
Delamination of paint is aggravated when the newly-painted wall is heated by sunlight. Free moisture retained within the plaster membrane (or concrete wall) which has been trapped within the system by paint, may vaporize and apply pressure against the film of paint from within the wall system. That internal pressure will lift paint from the surface of plaster or concrete in bubbles or blisters, or may cause the paint to curl outward at fissures in the substrate.
A secondary problem caused by too-early or premature application of paint to newly-applied cement plaster is that the plaster cannot receive the necessary water cure. Any water sprayed onto a painted wall would flow downward over the painted surface instead of being absorbed into the surface of plaster. Thus, the paint shield would prevent curing water from acting effectively upon the Portland cement in the plaster membrane, and would cause the wall to be soft and weak. Premature application of paint sometimes is encouraged or allowed by the developer, the builder, or the owner; or the painter may, of his own volition, elect to proceed too quickly, for his own convenience or economic gain. The person responsible for premature application of paint to new plaster must bear responsibility for delamination of paint, or for bubbles and blisters in the paint, or for curling of the coat of paint at breaks in the film, as well as for soft, weak plaster underneath the paint.
PREPARATION OF CRACKS, IF EXISTENT
Prior to application of the priming coat, any cracks in plaster or concrete should be filled. The filler should be a material which will withstand adverse effects of weather suitably well. If the plaster is textured, the painter MUST apply the sealant in a fashion which will match texture of the wall. Failure to accomplish that causes bands of smooth surface to be apparent through all successive coats of paint, to contrast distinctly and unfavorably with the adjacent field of textured plaster. Many painters are more aware of color and shading than of texture.
The primer and paint must be applied to the entire surface of the base, to have satisfactory results. After the primer has been applied and permitted to cure, the several coats of finish paint should be laid on sufficiently thick that uniform color is attained, and the desired decoration achieved. I sometimes encounter projects on which only one coat of paint has been applied, which is unsatisfactory, of course. I also encounter projects which have not had primer applied to the concrete or plaster in preparation to receive paint. Also, there have been projects on which it appeared that paint may have been thinned beyond the point of being an effective coat. It might be preferable that no paint be applied, rather than have a wall suffer inadequate coating.
Soluble mineral salts may be present in concrete or plaster, introduced by the portland cement, lime if present, aggregate, and mix water. Those soluble salts may remain inside the mass of the plaster or concrete, or they may migrate to the surface, in a manner described in my technical article entitled "EFFLORESCENCE". If efflorescence is present on the surface, it should NOT be disregarded by the painter. Instead, he should correct for the presence of the offensive material. Painters should be aware of the fact that efflorescence may form on the surface of concrete, cement plaster, concrete blocks and other products. Efflorescence may develop in various forms and types. Some efflorescence is locked firmly to the surface of the substrate, and may be difficult to remove. Other types are not bonded to the substrate and may be brushed away with a broom. In all instances, it behooves the painter to accommodate his procedure to the presence of efflorescence or any other deleterious substance on the surface of the substrate.
Most, if not all, paint manufacturers recommend that the proper primer be applied to new or relatively new concrete or cement plaster, to achieve desired bond, and avoid potential delamination of paint. Most paint manufacturers recommend that certain primers and paints should be used in conjunction with each other. It is my opinion that concrete and cement plaster always should receive a coat of primer prior to application of the paint coating per se. Unfortunately, concrete and plaster are not always primed by painters.
Paint may peel from the surface of relatively new concrete or cement plaster, especially if paint has entrapped moisture within the mass of concrete or plaster. The moisture may be present as carry-over mix water, or rain, or from some other source. The source is immaterial. The presence of moisture within the mass will cause an alert, knowledgeable painter to accommodate his procedure to fit the circumstances. Excess moisture must be allowed to escape from the plaster or concrete, as discussed earlier in this paper, or cleavage of paint from the substrate should be expected. All interested persons on a project should realize that concrete or plaster does not physically force a coat of paint to cleave from the wall. Instead, a rational person realizes that some force from within the mass acted upon the coating, unless the coat of paint had not bonded originally to the substrate. Note that paint delaminates from wood and metal substrates, as well as from concrete and plaster, if good preparation and application procedure have not been followed.
Paint applied to new cement plaster or concrete, before excess moisture has evaporated from the wall, may be expected to peel paint from the substrate. That deficiency is the fault of the painter, unless he was required by the builder to paint the surface too soon.
UNCONTROLLED SPATTER OR OVER SPRAY OR OVER-BRUSHING OF PAINT
Sometimes painters inadvertently, or through lack of knowledge, apply paint to the surface of the scratch coat or brown coat of cement plaster. In cases such as that, delamination of future coats of plaster may occur because of the paint acting as a bond-breaker.
Another, rare but observed, problem is that painters may be employed to paint a new plastered home. If the painters apply primer or finish coating prior to complete and proper water curing of the applied plaster, the plaster cannot be cured, and soft, weak plaster will have been created by the painter. That happens because paint that is applied to the surface of new plaster, before water curing of the plaster has been accomplished, serves as a shield to reflect water spray. Thus, water curing of a painted surface is ineffective, and the plaster behind the coat of paint will be soft and weak. That is so because paint kept water from contacting portland cement in the plaster membrane, to hydrate the cement properly. In that event, defective plaster is the fault of the painter.
In summary, cement plaster and concrete walls must be prepared to receive paint walls must be primed with the correct primer, paint must not be diluted, care must be exercised during application, timing of application must be controlled, and all other factors should receive careful attention.