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Exterior Wall Coatings and Treatment

This article discusses the application related issues in respect of exterior masonry walls while identifying the criteria that needs to be considered when assessing the suitability for a particular product and provides information on products currently available in the market.

A coating system that is used for exterior surfaces under the present context can be defined as a product in liquid, paste or powder form, that, when applied to a masonry surface or substrate, forms a film possessing protective, decorative and/or other specific properties.  It is the sum total of the layers of coating materials which are applied to a substrate. The purpose of a coating system is to impart preserving, decorating and protecting characteristics to new and old surfaces.

Coating materials and systems for exterior masonry surfaces are evaluated in terms of the substrate, the primary purpose in applying the coating and its ability to function satisfactorily over an economic period of time. Adhesion of the coating to the substrate not only depends on coating composition, but on the quality of substrate - primarily strength and texture, and substrate preparation if any. The thickness, (of a film or layer), of the coating, its long term compatibility with the substrate and its durability are factors that affect its capability of preserving the main function of the wall.

The resistance of an exterior masonry wall to rain penetration is primarily related to the design of the wall structure and workmanship in building the wall. Suitable coating systems provide that confidence that the wall is going to perform its intended function of keeping out the rain. Some walls such as cavity walls are inherently resistant to rain penetration. As such the coating system required is minimal. On the other hand, single leaf solid wall perform less efficiently thus the coating system is an essential barrier.

The substrate

The surface of application which here is considered as-Substrate, can be of concrete product, plastered surface, fairface finished concrete, brick masonry etc. It is also assumed here that the substrate is constructed as per prevailing standard norms.

Classification

Coating materials and coating systems may be classified in terms of their Primary function such as;

• Preservation
• Decoration
• Protection
• Waterproofing

Coatings are also classified based on Chemical type of binder;

• Hydraulic lime, silicate, cement
• Acrylic resin, vinyl resin, oil,
• Alkyd resin, (silicone), epoxy
• Resin, polyurethane and bitumen.

Classification based on Dispersion of the binder in emulsion or solution;

• Emulsion; water based
• Solutions; solvent based
• Solvent-free; e.g. epoxy

Masonry Finishes

Paints

Masonry Primer- A solvent based coating for priming new cement surfaces to provide an alkali resistant barrier as part of a paint system. It also has waterproofing properties depending on the make and quality. Some manufacturers have clear as well as pigmented masonry primers. Masonry primer should not be confused with so-called bonding liquids although many can fulfil a similar function. It is not designed as a finishing coat, but a cost-effective primer only.

• Emulsion Paints- These coatings are available in a wide range of qualities, solids content, high or low resin content and application rates; all giving various degrees of protection to the surface. However, there are some coatings which are custom made water proofers, made from specially modified acrylic emulsions for applying to problem walls that are already painted. Pure acrylics can be applied directly to a new plastered wall because they are resistant to alkali breakdown. Modified acrylics, PVAs, etc. require the surface to be primed with a good quality masonry primer. Filler coats are not recommended for external use because of their low resin content. If cracks require filling, non-chalking filler which is resilient and flexible should b used. Fabric tissue membranes can also be used together with the coating to reinforce the paint over cracks.
• A few cheap products available in the market show patterns of the substrate after getting exposed to rains due to moisture absorption and fail within months. The paint can even be washed or rubbed off the wall in extreme cases.

• Textured coatings- The texture will vary from quite light to very rough depending on the products. The texture in the product hides minor surface imperfections and ensures the paint goes on thick. This is particularly important to prevent water ingress into the substrate. In general these should be used to repair hairline cracks in walls as the course ingredients that give the product its texture will ensure the cracks are bridged. Generally the quality products also have additives that ensure flexibility in the coating. Thus the hairline cracks remain bridged. As with emulsion paints the price paid reflects quality. Textured coatings have a tendency to retain dirt.

Acrylates

These are solvent (or water) based clear coatings developed to fulfil a number of specific functions. They perform on their own and are not usually used together with other systems. Pigmented varieties are available for use in bathrooms, kitchens, etc. The clear products are used as decorative and waterproofing coatings for face work, etc. Its use as a carbonation barrier for off-shutter concrete results in the formation of a vapour barrier effectively trapping moisture in the substrate. (This is not recommended for masonry used for habitation.) Its excellent UV resistance make it suitable for preventing dirt ingress and fungus growth in paving. The user should be selective in his choice of these coatings because some types turn a milky colour in the presence of water.

Silicones

Silicone is the name of a family of products: Siloconates, Silicone Resins, Silanes and Siloxanes. The use of each is fairly specific. They all, however, have a number of characteristics in common in respect of how they function on a masonry surface:

• They are thin liquids which penetrate into the substrate thereby lining the capillaries and stopping capillary absorption of water. It is therefore not a surface coating but a substrate treatment. A simple flooding run-down application achieves the desired penetration.
• Their ability to repel water ingress is excellent in walls in which all holes and cracks are repaired. Where crack widths exceed 0,3mm, the effectiveness of the Silicone against water penetration is greatly reduced.
• They are UV-resistant.
• The treated raw masonry surface is paintable as paints contain wetting agents.
• These should only be applied to unpainted masonry (concentrated silicone on painted surface will repel further coats of paint).
• They effectively prevent all water-caused damage to masonry such as, efflorescence, washing out of lime, moss and fungi growth, peeling of interior paint, corrosion of structural steel, damp patches after rains, etc.
• Some silicone products are available in ready-to-use form whilst others are concentrates requiring dilution with the correct solvents.

Cementitious wash

These products are available in powder form. They are basically comprised of white or grey cement to which synthetic resin binders and pigments are added. These binders improve water resistance, adhesion and serve as a carrier for the pigments. Cementitious washes contain chemical complexes which combine with free lime in the concrete masonry substrate and therefore improve adhesion of the product. However, surface preparation is critical and care has to be taken in the application for successful results. The manufacturer instructions should be followed on the application of the product. Cementitious washes have the ability to cover imperfections on the masonry. The powder is mixed with water to slurry and applied by block brush in two or three coats. Available in the variety of colours, although some are grey cement-coloured.

Plasters

Plaster additives These are liquid additives which replace some of the water in the mortar mix. Their action is to retard the penetration of water to a greater or lesser degree depending on the quality of the product and chemical action. The higher-priced products are more effective.

Resin Based Plaster coatings- These resin bound plaster coatings are trowel-applied to a primed raw plastered surface to obtain special decorative effects on upmarket properties.

Maintenance of masonry walls

• During construction, particular care needs to be taken to prevent mortar smears on the face of the masonry unit. Mortar droppings which adhere to the exposed face of a unit can be removed with a chisel after being allowed to dry and harden. The remaining mortar can then be removed with a stiff fibre brush.

• Cleaning of unpainted walls can usually be done by scrubbing with water and a small amount of detergent. Clay or dirt should first be removed with a dry brush. Most efflorescence can be removed when it first appears by simple hosing with water but some efflorescence requires brushing with acid. One part hydrochloric acid and nineteen parts waters is a good concentration - adequate for the job and not too disagreeable to work with.

• The wall should be wetted before the diluted acid is applied. Care must be taken to prevent splashing on floors and other surroundings. The wall must be rinsed thoroughly afterwards with a hose if outdoors, or several times with buckets of clean water and sponges if indoors.

• Rubber gloves and goggles should be worn. Since the acid wash will probably create a subtle change in the appearance of the surface with which it comes in contact, care must be taken to wash the entire wall to avoid undesirable variations in appearance.

• Maintenance of the well-designed and constructed masonry wall is minimal. On unpainted masonry wall is minimal. On unpainted masonry surfaces, however, inadvertent staining from oil, grease or other foreign substances can destroy the appearance of an otherwise attractive structure. Stains resulting from metal or other attachments to the wall can also create appearance problems.

Reference:

Concrete Manufacturers Association Manual