High Performance Maintenance Coatings

COATING SELECTION CRITERIA

Before proceeding, a discussion of some of the more important coating selection criteria is in order. Many of these factors are obvious; others are more subtle but still critical to overall coating performance.

Interior or Exterior Application

Exterior coatings are designed to resist more severe conditions that interior products. Weathering pigments and fillers are costly and are included in exterior formulations for their resistance to breakdown by sunlight and other outdoor conditions. One of the common errors is to use an interior grade product which is subjected to intense sunlight from windows or skylights for a significant part of the day. Discoloration will quickly occur in these areas, leading to any unsightly aesthetic condition.

Vertical or Horizontal Surface

Coatings are formulated to possess different levels of resistance to sag or flow. Obviously, the greater the sag resistance, the greater the slope on which the coating can be applied successfully. A material formulated for horizontal surfaces will produce runs and sags even in thin coats if applied on a vertical. Besides the resulting poor appearance, it would be impossible to obtain the coating thickness necessary for proper hiding and performance. Coating coverage rates are determined by carefully testing various thickness levels to find the correct combination of performance and finished job cost. It is a mistake to interchange vertical and horizontal grades of product, as the results will be poor product performance and unsatisfactory appearance.

Chemical Resistance

This property encompasses a wide variety of conditions and exposure. The point of listing this criteria is to emphasize that any unique of unusual performance requirement should be considered in the coating selection. For example, the application might be near manufacturing facilities so that the coating must be resistant to acidic or caustic environments. A swimming pool area would require the use of materials with good performance in highly humid conditions. Appropriate products would be required in kitchens where there is grease and oil spillage and splashes. Different chemistries and curing mechanisms can be formulated into the coating to build resistance against specific chemical agents and deteriorating surroundings. This information should be furnished to the coating supplier to ensure the proper product selection.

Temperature

Certain types of coatings have much better resistance in constant conditions of high or low temperature. Silicone materials, for example, do much better in applications involving exposure to high temperature for lengthy period of time. Elastomerics are the preferred choice in very cold environment. Cold temperature flexibility or resistance is one of the key formulating parameters used in determining the projected service life of a coating as it reflects on other properties, such as fatigue resistance, elongation and stress relief.

Odor and Safety

Odor can be a serious issue if the occupants of a building or facility are unfamiliar with or sensitive to the solvents or other volatile ingredients in paints or coatings. Even latex- or water-based products have some degree of odor due to plasticizing agents, minor amounts of solvents or other additive materials.

With today’s heightened awareness of safety and health issues, a thorough evaluation should be performed before any painting or coating applications are started.

Odor should be anticipated in any painting or coating project, with steps taken to minimize the impact. Projects can be performed on weekends or during vacation periods. At minimum, personnel can be alerted to any work that will take place and be assured that the odors present no health risk. With today’s heightened awareness of safety and health issues, a thorough evaluation should provide information regarding any health risks posed by the materials being used. Material Safety Data Sheets should be obtained on all products involved with the job. If a contractor is performing the application, it should be demonstrated that all personnel are using proper safety procedures and wearing the protective equipment necessary for the particular materials being applied.

VOC and VOS Regulation Compliance

Both the terms VOC (volatile organic compounds) and VOS (volatile organic substances) apply to the materials which evaporate from an applied coating during the curing process. Several states and localities now have regulations which significantly limit the amounts of organic coating components which are permitted to volatilize into the air.

These regulations list different levels or limits for different categories or coating materials. For example, roof coating have particular VOC or VOS limits, while floor coatings and paints have different levels or allowable emissions. When performing re-coating work, the applicable regulations should be determined and the coating manufacturer should verify that materials being supplied comply with these permissible limits. Many coating suppliers are now listing on container labels the levels or organic materials which evaporate during the drying process. Regulations are in a constant state of change, and several different forms of these rules are now being used. Be sure to contact local code officials to determine the specific limits being set for a particular building or facility.

Aesthetics

This is a broad area which includes a wide variety of performance and perception issues. Performance considerations, such as color and gloss retention, resistance to chalking, as well as the ability to resist cracking under movement conditions, all enter into the assessment of a coating’s appearance. On the other hand, the range of colors available, resistance to dirt pick-up and cleaning characteristics are more related to a material’s decorative qualities.

Cost

It is important that the correct quality or performance level of product be matched to the requirements of the job. Using an expensive product for an application requiring minimum performance is, of course, a waste of money. On the other hand, employing a product not capable of performing adequately for the particular end-use is also a foolish expenditure. The result of suing an inferior product is simply that the work will have to be re-done in a very short period of time.

Many other criteria enter into selecting a coating product. No list can be all-inclusive, as many factors are specific to the job. Food processing or storage facilities, for instance, require federal government approvals as proof that the coating will not taint or contaminate through incidental contact. Some thought should also be given to future maintenance and re-coating needs. Is the coating to be applied compatible with a variety of other products? Some materials have very poor adhesion properties when any type of coating, including the same product, is applied as a recover coat.

Concrete and Masonry Surfaces

In the past, the term “masonry” referred to a building component constructed by a mason using individual units of stone, brick or tile. However, the term now includes almost any material using concrete or plaster as one of its components, ranging from brick and block walls to precast concrete, asbestos-cement board, stucco, plaster and many other building products.

The performance of coatings on masonry surfaces is affected by properties unique to this type of material. Four that will be briefly mentioned are:  bulk properties, such as porosity, which results from the use of water during the forming and curing process; surface texture or smoothness; alkalinity; and the presence of soluble salts.

The porosity of masonry materials can create problems in that, even when all the moisture used in production or application is removed, sites remain for the potential absorption or passage of water. With some masonry products, the pores are so minute that only very small molecules of water can pass through. The much larger molecules of a coating cannot physically fit in these pores, and therefore, cannot physically fit in these pores, and therefore, cannot prevent moisture from entering the masonry.

The characteristic of surface texture can be a problem if the masonry is too smooth or too rough. If the trowel finish is excessively smooth, there will be no sites for the coating to key into, resulting in poor adhesion. Glazed brick or tile and polished stone also present serious problems in this area. On the other hand, if the masonry finish is very rough, large holes can be formed on the surface and the fluid coatings cannot form a continuous film.

Most masonry is highly alkaline in nature. Although brick and stone are normally neutral, the cement or mortar in which they are set is alkaline. This condition of alkalinity can result in serious coating deterioration. If the selected coating contains ingredients which readily react with alkalies in the masonry, a chemical process known as “saponification” occurs. The coating becomes soft and tacky under these conditions and, if the reaction proceeds to a significant extent, will actually liquefy. There are three approaches to prevent this saponification reaction. The first and most obvious is to use a coating material which does not react with the alkali material generated by the masonry. The second is to use a primer which is resistant to this reaction and acts as a barrier between the coating and alkalinity. A third approach is to neutralize the alkaline surface prior to applying the surface coating. The problem with the last solution is that if water passes through the masonry, it can bring fresh alkali to the surface.

The presence of soluble salts in masonry leads to a phenomenon called "efflorescence.” These salts are carried to the surface and result in the white staining often seen on brickwork and similar materials. The formation of efflorescence is severe enough to destroy coatings place over it. Further, simply removing efflorescence prior to coating may not be sufficient. If moisture does not cease to enter the masonry, the efflorescence will continue to form, leading to destruction of the coating film. The only permanent solution is to eliminate moisture entrance. Usually, this involves repairing some type of structural or design problem.

Two other problems associated with masonry and coating applications are improper drying of the masonry after installation and the presence of surface agents which may have been used during manufacture or installation. Placing a solvent-based coating over wet masonry will, or course, result in adhesion problems. Water-based coatings will not be severely affected by damp surfaces, but excessive moisture will prevent proper cure and result in poor film integrity. A variety of chemical treatments may be used on masonry materials. Many form-release agents contain waxes or oils. This will lead to poor adhesion with water-based coatings, as well as drying time and adhesion problems with solvent-based products. Other treatments, such as “cure and seal” products or damp-proofing materials, may also result in poor coating adhesion or some other type of incompatibility with coatings. It is recommended that new masonry not be coated for approximately 6 to 12 months to allow the treatments to weather. Alternatively, any surface agents can be removed by some form of abrasion.

Weather conditions at the time of application can also have a profound effect on the coating’s long-term performance. For example, porous surfaces will absorb water during rainfall or when exposed to high humidity. This absorbed moisture will interfere with adhesion and affect the cure rate of all the typical materials used as coatings. Very low or high ambient temperatures will substantially slow and sometimes even permanently prevent the cure of coating materials. Very high temperatures will interfere with proper application and may result in stress cracking because of excessively fast cure.

Among the recommended masonry coatings, latex or water-based paints have given excellent result over the years on concrete, stucco and brick surfaces. Due to the porosity of these masonry materials and their hydrophilic (water affinity) characteristics, latex products have offered superior performance. One concern which must be mentioned is that latex paints should be re-coated before sever chalking occurs. Latex paints do not “wet out” a chalked surface as well, and coating this type of surface would lead to poor adhesion. If a latex paint surface has chalked, it should be thoroughly cleaned prior to repainting. An alternate solution would involve the use of a penetrating solvent-based sealer prior to applying the latex paint.

The desirability of painting brick has been debated over the years. Much of the negative results obtained when painting brick surfaces can be explained. In some cases, the selected coating did not have sufficient performance and peeled or flaked off. In other instances, latex paints were applied over water-repellent materials and simply did not wet or adhere to the surfaces. A third problem was the use of coatings with little water permeability. Any water trapped in the brick structure would go through freeze/thaw cycles and result in spalling or flaking of the brick surface.

Before applying a breathable latex paint to a brick structure, it should be determined that water-repellant treatments have not been used. This is done by applying a small test patch of latex paint or simply determining if water will be absorbed by the surface rather than forming droplets. It is, or course, desirable to make any repairs to the motar and clean the surface thoroughly prior to coating brick structures.

Solvent-based coatings can also be used on masonry surfaces, providing they offer some “breathing characteristics.” These coatings are usually formulated with fillers which allow for water vapor to pass through. This relieves the problems created when there is a moisture drive in one direction or another.

One last comment on masonry materials involves asbestos-cemented surfaces. Although not in common use anymore, there still is a great deal of this material in place. These products tend to absorb moisture very readily, and coating them must be done with great care. If an impermeable coating is used on one side only, they will warp and bow. Water will be absorbed on the uncoated side in moist conditions causing expansion, and the uneven forces will lead to dimensional instability and distortion.

Coating Wood Substrates

As with any coating job, selection, proper preparation and correct application are the keys to obtaining the best finish and optimum performance on wood surfaces. It is interesting to note that under typical conditions, coatings are not essential to protect wood from the effects of weather. Exposed wood will turn a natural gray color if frequently wetted and will become reddish brown in a very dry climate. Unprotected wood will, however, experience some checking or cracking because of the process of wetting and drying when exposed to direct sunlight. Additionally, wood will rot or decay if fungal growth is allowed. Since fungus is developed under damp conditions, any steps which protect the wood from moisture will significantly reduce the chance of deterioration.

When selecting the proper coating for new wood, it is important to understand that significantly different quantities of materials will be necessary for different types of wood.

When selecting the proper coating for new wood, it is important to understand that significantly different types of wood. This is because the porosity of various woods may vary substantially. Environmental conditions are especially important in coating selection for wood surface. It is also key to understand that wood changes dimensions during drying and wetting cycles. Changes of 2 percent due to contraction and expansion are typical, and coating must have sufficient elongation throughout their service lives to deal with this dimensional change.

In general, paints and coatings for wood are placed into two overall categories:  water-thinned or organic solvent-thinned. Latex paints are water-thinned and are popular choice. They offer easy clean-up with water and, generally when all factors are considered, are preferred over the solvent-based products. Latex paints dry more quickly and exhibit better floss and color retention. Additionally, they show superior mildew and blister resistance.

Solvent-based paints, also known as oil/alkyds, serve as good moisture barriers. They also have excellent adhesion to most substrates and can be applied at lower temperatures than the latex products. Both latex and solvent-based paints are available in an unlimited number of colors and a variety of sheens, from flat to high gloss.

Primers are available in both types of coating and are formulated to provide a sound base for subsequent coating applications.

Surface Preparation and Application

Before application of any paint or coating, the surface must be thoroughly prepared. Any loose paint of coating must be removed by sanding or careful scraping. Typically, no attempt should be made to remove any primer or penetrant that has been applied previously to the wood. When coating over glossy paints or non-weathered areas, it may be necessary to scuff the area with fine sandpaper before coating. Surfaces should be cleaned of any dust or dirt by washing with a detergent solution using a pressure sprayer or scrub brush, then rinsed with clean water. Check for the formation of mildew.

Typically, mildew can appear as dirty streaks on the surface of the paint. It is a natural fungus growth cause by spore in the air which attach to the coating and must be removed before coating or recoating. The presence of mildew can be confirmed by a simple test. One drop of 5 percent sodium hypochlorite solution (common household bleach) should be applied to the stain. Usually, mildew will be bleached (discoloration removed) in one or two minutes. A stain that does beach is usually dirt. To remove mildew, a commercial mildew has appeared on a certain area, it is strongly suggested that a mildew-resistant paint or coating be applied.

In any paint or coating application, it is critical to follow the manufacturer’s directions carefully. The manufacturer’s label instructions will cover specific recommendations regarding surface preparation, spread rates, proper application tools and conditions, as well as a variety of other areas for correct use of the product. The first coat of primer or paint should be applied by brush. This drives the coating or paint into the substrate, allowing the material to penetrate into the surface and form a strong, long-term bond with the wood. Subsequent coats can be applied by any conventional means. Regardless of the method of application, follow the coverage rate recommended by them manufacturer. Finally, it is important to work during the right weather conditions. Do not pint surfaces with latex formulations on very hot and humid days or when the temperature may fall below 50°F. No coating or painting should be done when rain or fog is anticipated.

Metal Roof and Wall Systems

Applications for metal in building construction can be placed in four categories:  roofing, cladding and flashings; structural and reinforcing purposes; in building services, such as piping, storage tanks, drains, ducts and similar; and lastly, in underground applications.

Undoubtedly the most important problem associated with metals is corrosion and its inherent consequences. Metals which are subject to atmospheric conditions are most prone to corrosion. Since corrosion is promoted by the presence of water and electrolytic salts, metals which are protected by the exterior shell of the building require less protection than those exposed to the elements. The increase of air pollution has, or course, intensified the corrosion of metals because of the increase in contaminant salts.

The focus of this discussion will be on the rusting of steel and on protection methods for exterior applications. Steel is the most often-used metal in buildings because of its low cost, strength and versatility. Additionally, emphasis will be place on applications where steel panels are used for wall cladding or roof panels. The points made, however, are appropriate to most other exterior steel applications, such as storage tanks, sheds, etc.

Analyzing the Problem

The problems associated with steel wall cladding and roofs requiring remedial treatment are:  fasteners; seams projections or penetrations; rooftop equipment; rust of corrosion; and failed coatings. After deciding on the type and extent of the problems which exist, several maintenance options are available. These include:  paints or

rust-preventative materials; coatings offering some waterproofing; and systems containing several products to handle a number of existing problems.

Paints

Paints have little or no waterproofing capabilities by themselves. They serve predominately to protect the surface of the substrate to which they are applied. They also provide at least three other functions when used on exterior metal surfaces. Many paints are formulated with rust-inhibiting chemicals to forestall the formation of new rust and to neutralize rust which already exists. Sometimes, these rust-inhibitors are incorporated into a primer for greater penetration into the rusty surface. Paints also improve the aesthetics of a wall or roof. This is the most obvious result of a painting application. Light-colored or aluminum paints create a reflective surface which results in reduced temperatures in the building interior. This can lead to substantial energy savings in areas of the country with significant air conditioning usage.

There are many products available for use on rusted or non-rusted metal surfaces. Each product has its own unique chemistry, and there are several approaches now in use to prevent or treat rust. Alkyd rust paints, zinc oxide primers, one and two-part epoxies, aluminum-pigmented materials and various types of rust conversion products are all used to coat exterior metal surfaces.

As with all paint applications, proper preparation of the surface is key. All loose rust must be removed using some type of abrasion, and the substrate must be clean for good adhesion. Also important is that any existing coating is well-adhered and compatible with the material to be applied. It is always good practice to place a test patch to check for characteristics such as adhesion, bleed-through and any effects on the existing coating.

Repeating an earlier point, do not attempt to correct leaks using a paint applications. Paints can lead to a number of positive results. However, any reduction of leaks will be temporary unless the proper preparation and products are used before the paint is applied.

Coatings for Metal

Coatings are more viscous or heavily-bodied than paints. As a consequence, they offer a greater degree of waterproofing and can be applied at a much heavier application thickness. Usually, coatings have somewhat better mechanical properties compared to paints from the standpoint of elongation, recovery, fatigue resistance and flexibility. Coatings will seal pinhole leaks, and because of their improved properties, will maintain a water seal for some period of time. They are, however, not a permanent solution to a leak problem. Because of the great movement of metals, even coatings will fail over joints, fastener heads and other areas of metal construction prone to problems.

Coatings include materials such as asphalt-based aluminums, standard and modified asphalt emulsions, latex-based products and rubber-based solvent materials. Coating perform all the functions of paints but are longer-lasting and have additional waterproofing characteristics.

Maintenance Systems

The most comprehensive approach to treating construction metals is the use of maintenance systems. They consist of a number of different products to treat all the usual problems.

Maintenance systems include primers which are formulated to be low in viscosity to guarantee rust penetration and sealing. The primers have rust inhibiting additives which can be sophisticated enough to react chemically with the rust to form stable compounds. These systems also include paints – which are usually less costly than coatings – for areas which only require the type of protection offered by these materials. Coatings are also available and are employed when the additional waterproofing is necessary or a longer-term application is desired.

Perhaps the key products used in maintenance systems are sealants and mastics. They are bulked versions of the coatings and can be sued at much heavier rates of application. Sealants/mastics are packaged in cartridges and cans and are formulated to be applied by trowel, brush or even spray guns. They are used to stop leaks. Because of their viscosity, a volume of material can be placed to ensure that they create a substantial barrier to water penetration. They are used most commonly is seam areas, over fasteners or in areas where a substantial amount of movement occurs.

Often, these products are used in conjunction with some type of reinforcing fabric or scrim. The reinforcement is placed between two layers of the mastic or sealant to provide greater integrity and strength to the repair. The fabric acts similarly to fibers incorporated into the composite materials used in boat hulls and other high technology applications. Many of these system manufacturers are now using some form of polyester as the reinforcing mat. Polyester offers outstanding strength as well as resistance to tear, puncture and fatigue. Glass mats are also used with metal repair systems.

Sealant and mastic products can be formulated from a number of elastomeric materials. Acrylics, urethanes, silicone and butyls and SBS (styrene-butadiene-styrene) rubbers are just a few of the polymer materials which can serve as a base for these sealants and mastics. Often the coatings are formulated with the same polymers to ensure compatibility in the system.

Repair Tapes

It is important to mention the use of repair tapes with regard to metal maintenance. They can be viewed as pre-manufactured coatings. To avoid the necessity of the mastic-scrim application mentioned earlier, tapes are often used. They are self-sticking and incorporate scrim or mat. Polyester and polyethylene are two of the more common reinforcements used in tapes. The advantage of tapes, besides easier installation and less dependence on the skill of the mechanic, is that they are cured when applied. There is no need to wait for cure to take place, and the application can proceed immediately.

This overview has covered a few of the problems associated with the most commonly-used building materials. It should be evident that most can be resolved through a basic knowledge of materials, plus prompt and proper remedial actions. After the problem is corrected, maintenance programs can prevent reoccurrence and usually provide significant cost paybacks on a long-term bases. It is also important to understand that proper selection and application of coatings and other treatment can specifically address typical material aging effects and produce excellent appearance as well.

Originally published in AIPE Facilities, January/February 1993