Masonry Magazine May 1972 Page. 23
FILL COATS
Fill coats are base coats for exterior masonry. They are similar in composition, application and uses to cement-based paints. However, fill coats contain an emulsion paint in place of some water, giving improved adhesion and a tougher film than unmodified cement paints. Fill coats have greater water retention, giving the cement a better chance to cure. This is particularly valuable in arid areas where it is difficult to keep the painted surface moist during the curing period.
SOLVENT-THINNED PAINTS
The five major solvent-thinned paints are oil- based, alkyd (synthetic resin), synthetic rubber, chlorinated rubber and epoxy. Oil-based and alkyd paints are not recommended for exterior masonry. Solvent-thinned paints should be applied only to completely dry, clean surfaces. They produce relatively nonporous films and should be used only on interior masonry walls not susceptible to moisture penetration. The exception to this is special purpose paint, such as synthetic rubber, chlorinated rubber and epoxy paints.
Oil-Based Paints. Oil-based paints have been used for many years. They are relatively non- porous and recommended for interior use only. Although several coats may be required for uniform color and good appearance, they bind well to porous masonry. As with most solvent-based paints, they have good penetration on relatively chalky surfaces, but are highly susceptible to alkalies. New masonry must be thoroughly neutralized to avoid saponification. Available in a wide color range, oil-based paints are moderately easy to apply. Several days drying is generally required between coats.
Alkyd Paints. Alkyd paints are similar to oil- based paints in most general characteristics. They may have slightly less penetration, resulting in somewhat better color uniformity at the cost of adhering power. Alkyd paints are more difficult to brush, dry faster and give a harder film than oil-based paints. These, too, are nonpermeable and are recommended for interior use only.
Synthetic Rubber and Chlorinated Rubber Paints. These paints have excellent penetration and good adhesion to previously painted, moderately chalky surfaces as well as new surfaces. They are reported to be more resistant to efflorescence and are generally good in alkali resistance. They may be applied directly to alkaline masonry surfaces, but are more difficult to brush on than oil paints. Darker colored synthetic rubber paints lack color uniformity. Both types have high resistance to corrosive fumes and chemicals. For this reason, they are often specified for industrial applications. Both types require very strong volatile solvents, a fire hazard which may prove undesirable.
Epoxy Paints. Epoxy paints are of synthetic resins generally composed of two parts, a resin base and a liquid activator. They must be used within a relatively short time after mixing. Epoxies can be applied over alkaline surfaces, have very good adhering power, and good corrosion and fume resistance. However, some types chalk excessively if used outdoors. Epoxies are relatively expensive and somewhat difficult to apply.
"HIGH-BUILD" PAINT COATINGS
High-build paint coatings are generally used on interiors to give the effect of glazed brick. Some coatings are based on two-component urethane polyesters and epoxies. Others are of an emulsion-based coat with acrylic lacquer. These paint systems usually include fillers to smooth out surface irregularities.
OTHER COATINGS
Heavily applied coatings of the so-called "breathing type" are available with either a water or solvent base. They are generally composed of asbestos fiber and sand, and applied thickly to hide minor surface imperfections. The presence of moisture on the surface of a masonry wall generally will not harm the latex type. Lower application temperatures of 35 F to 50 F on the other hand are less damaging to the solvent type.
For both types, adhesion is mostly mechanical because of low binder and high pigment content. Some coatings require special primers to insure adhesion. Although these coatings are reported to have given good performance on masonry, they tend to show stains where water runoff occurs.
These coatings are capable of allowing passage of water vapor, but cannot transmit large quantities of water that may enter through construction defects. Failure may occur as a result of freezing of water accumulation behind the film.
PAINTING NEAR UNPAINTED MASONRY
Often windows and trim of masonry buildings are painted with self-cleaning paints to keep surfaces fresh and clean. Unfortunately, self-cleaning is generally achieved through chalking. The theory is that rain will wash away chalked paint, constantly exposing a fresh paint surface. The theory works well, but too often no provision is made to keep chalk-contaminated rain water away from masonry surfaces. The result is usually more unsightly than dirty paint on trim or windows. Avoid this staining by choosing nonchalking paints for windows and trim and by providing a means of draining water away from wall surfaces.