The treatment of exterior masonry surfaces with coatings has the purpose of preserving, decorating and protecting new and old, coated and uncoated masonry. 

This article addresses the function of a coating or a coating system to prevent rain penetration through a masonry wall. It also identifies criteria that need to be considered when assessing the suitability for a particular product and tips on maintenance. 

The treatment of exterior masonry surfaces with coatings preserves, decorates and protects new and old, coated and uncoated masonry. Image credit: Berts Bricks

Evaluation 

The primary purpose is 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 the coating composition, but also on the quality of the substrate – primarily strength and texture, and substrate preparation if any. 

The thickness of the coating, its long-term compatibility with the substrate and its durability are also factors that affect its capability of preserving the main function of the wall. 

The resistance of an exterior masonry wall to rain penetration is also related to the design of the wall structure and workmanship in building the wall. Suitable coating systems provide confidence that the wall is going to perform its intended function of keeping out the rain. 

Masonry finishes 

This reference guide covers paint, acrylates, silicones, cementitious wash and plasters.  

Plasters can include liquid additives, which replace some of the water in the mortar mix to retard the penetration of water.

Paint 

Acrylates 

During construction, particular care needs to be taken to prevent mortar smears on the face of the masonry unit. Image credit: CMA

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 facework, 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. 

Acrylates’ excellent ultraviolet (UV) resistance makes it suitable for preventing dirt ingress and fungus growth in paving. The user should, however, be selective in the choice of these coatings, because some types turn into 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. However, they all have a number of characteristics in common in respect of how they function on a masonry surface: 

• They are thin liquids that 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 rundown 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. 
• UV-resistant. 
• The treated raw masonry surface is paintable as paints contain wetting agents. 
• Should only be applied to unpainted masonry, as concentrated silicone on painted surfaces will repel further coats of paint. 
• Effectively prevents 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 and damp patches after rains. 
• Some silicone products are available in a 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 that combine with free lime in the concrete masonry substrate and therefore improve adhesion of the product. 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.  

These products have the ability to cover imperfections on the masonry. The powder is mixed with water to a slurry and applied by block brush in two or three coats. Cementitious washes are available in a variety of colours, although some are grey cement-coloured. 

Plasters 

Graffiti-resistant coatings 

Untreated masonry is generally absorptive and susceptible to graffiti and staining. If vandalism is a possibility, then a graffiti-resistant coating should be considered. 

Coating materials would resist the absorption of marking into these surfaces and can easily be cleaned. Coating of this kind would not only simplify removal, but substantially reduces the cost of building maintenance.  

Untreated masonry is susceptible to graffiti and staining. A graffiti-resistant coating should therefore be considered where vandalism is possible.

Maintenance of masonry walls 

The maintenance of a well-designed and constructed masonry wall and unpainted masonry walls 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 problems. 

Mortar smears 

During construction, particular care needs to be taken to prevent mortar smears on the face of the masonry unit. Mortar droppings that 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. 

During construction, particular care needs to be taken to prevent mortar smears on the face of the masonry unit. Image credit: CMA

Cleaning 

Unpainted walls can usually be cleaned by scrubbing with water and a small amount of detergent. Clay or dirt should first be removed with a dry brush.  

Efflorescence 

Most efflorescence can be removed when it first appears by simple hosing with water, but some efflorescence requires brushing with an acid solution.  

Since an 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. 

Graffiti 

A good cleaning material should be quick-acting and easily remove the marking material without damaging the substrate. Strong chemical cleaners, however, may easily etch the masonry.  

In many cases, traces of the markings are invariably left behind as shadows, because pigments and binder have penetrated into the pores of the masonry. High-pressure water-blasting or steam-cleaning using strong detergents may prove effective with some markings. 

These methods are expensive and may not be completely effective. It can damage the surface of the masonry and other areas. Dry and wet sandblasting methods are also very effective but may do even more damage. 

Issue: Protecting masonry walls. 

Solution: This reference guide discusses how to evaluate products, explores different types of coatings and gives tips for maintenance.  

Full acknowledgement and thanks go to http://www.cma.org.za for the information in this article.  

For more information, contact the Concrete Manufacturers Association (CMA):  

Tel: +27 11 805 6742 

Email: admin@cma.org.za 

Website: www.cma.org.za