Embarking on a school painting project is not as easy as rushing off to the hardware store to grab some paint and brushes, running back to the school, dipping the brush in and watching the color appear on the wall. Paintings and coatings literature can be filled with jargon meant for chemists, and it's not always easy to decipher exactly what you need for which walls. In addition, schools have many types of surfaces that require a well-stocked paint inventory.
Typically, a coating system consists of the surface preparation, primer, undercoats and a finish coat. Ideally, a coating system should be designed with the following key considerations:
What is the budget for the paint or coating project? Different systems have different costs, and performance usually is directly proportional to cost.
Wood, steel, galvanized steel, aluminum, masonry and concrete have different properties and usually require different paint systems.
Marine, inland, chemical, interior, exterior and commercial locations have different corrosive properties and require different systems to perform satisfactorily.
- Life expectancy
How long is the system expected to last before beginning routine maintenance coating and before complete recoating is required? Life expectancy usually is directly proportional to cost.
To a lesser extent, the availability of materials and the skill of the applicators should be evaluated when selecting a coating system.
What's in a pail of paint?
Just as a play has actors with specific roles, a bucket of paint has components that play specific roles. These components are resins, pigments and solvent. Their roles:
- Binders (Resins)
Resins play the main role in a bucket of paint. Resins are the solids that bind the other ingredients together and determine the coating's adhesion. Binders form the film after curing. Binders that use water as a solvent are called water-reducible. Similarly, binders that use a solvent are solvent-reducible. Technology with regard to water-reducible paints and coatings has improved significantly in recent years, and their performance is virtually equal to that of the solvent-reducible binders. With the advent of stronger restrictions from the Environmental Protection Agency (EPA) for the release of volatile organic compounds (VOCs), water-reducible binders have become more popular and are increasing in market share.
Pigments provide a supporting role and are the other solid component of the pail. Pigments include additives to provide color and special performance characteristics such as resistance to sag, mildew and bacteria. However, their primary purpose is for color. Lead, a pigment once used to prevent corrosion and improve flexibility, now is prohibited from formulations, but 30 to 40 years ago, red lead primers probably were the predominant primer for corrosion resistance. Special precautions must be used when removing old lead-based paints.
Solvent is another supporting component and sometimes is called “reducer,” “thinner” or “carrier.” It keeps the binder and pigment components in a liquid form and allows the coating to solidify during evaporation. The type of thinner in a paint or solvent determines whether it is water-based or solvent-based. The solvent in solvent-based coatings, such as oil, alkyd, catalyzed epoxy and catalyzed polyurethane, is an organic solvent. Different paints and coatings use different solvents, and using the wrong solvents usually will destroy performance. Mixing different solvents by using old solvents left over from previous projects courts disaster. Always refer to a manufacturer's product data sheet for a particular product to find out the correct solvent.
Taking an inventory
Following are some common types of coatings used for initial and maintenance coatings. A typical maintenance paint inventory may contain several different types of paints, possibly by different manufacturers. Although it may be tempting, do not “cross-coat,” or mix a paint from one manufacturer with paint from another manufacturer either in the bucket or on the substrate. Manufacturers formulate paints and coatings to work in synergy with their own products, and mixing can lead to trouble. Even if one manufacturer's paint is applied to another manufacturer's existing paint, it is risky because of determining responsibility in case of a failure.
Oil-based coatings are a predecessor to alkyd coatings and usually are based on linseed oil, tung, castor or fish oils. Typically, they dry slowly, are soft, and offer little or no chemical resistance. These are suitable to rural and mild urban environments and have a tendency to harden. Today, when oil-based paint is used, it is usually for residential applications.
Alkyds have replaced oil coatings and have been the workhorse of coatings for years. Alkyds are solvent-based coatings that are made with polyester resins. They cure by the evaporation of the solvent and by oxidation of the resin. Alkyds often are classified as long oil, medium oil and short oil. They have reasonably good film buildup to 3 mils (DFT) and can be used in mild industrial and lesser environments. However, they should not be exposed to strong acids, alkali or solvent conditions. Applying alkyds over concrete, masonry or galvanized steel without an alkali-resistant primer over the substrate may result in saponification, which will cause delamination. Even using an alkali-resistant primer is risky because of the reliance on a thin barrier coat placed between the substrate and the alkyd. Saponification is the formation of soap from a reaction of an oil with an alkali.
- Silicone alkyd
These are modified with silicone and have properties similar to alkyd, except that they have much better color and gloss retention. They have performed well in coastal areas and areas with strong sunlight.
Latex paints are water-soluble and dry by solvent evaporation. They are made from synthetic resins and include acrylics and vinyl acrylics. Acrylics, especially 100 percent acrylics, have excellent color and gloss retention in exterior applications, and have greater adhesion than oil and alkyds. Because acrylics have improved performance and low VOC content, and are easy to apply and clean up after, they are replacing oils and alkyds in both initial and maintenance painting.
Epoxy coatings are all-around, high-performance coatings that offer good chemical resistance. They are dependable, provide excellent adhesion, and are available in catalyzed, single-component, solvent-base and water-base formulations. Many will meet VOC requirements.
- Catalyzed epoxy
These are two-component coatings consisting of an epoxy resin and a hardener. They cure from solvent evaporation and a chemical reaction called copolymerization. Epoxy coatings have been around a long time, and there are many variations, with each variation assigned a special role. The most common and versatile epoxy coating is epoxy polyamide. It offers excellent adhesion to most substrates, and resistance to water, weather and chemicals. It tends to yellow and chalk when exposed to UV or fluorescent lighting. However, the chalking and yellowing are aesthetic issues and will not affect performance. Epoxy can perform better when used as a primer or undercoat, with a polyurethane as a finish coat. Epoxy is available in high-solids, solvent-based formulations that meet VOC water-based formulations.
- Single component epoxy
Like the alkyds, single-component epoxy cures by solvent evaporation and oxidation. Toughness and chemical resistance of single components are not up to those of catalyzed epoxy, but are better than those of alkyds. Epoxy Ester is a typical single-component epoxy.
- Catalyzed epoxy
These are hard, chemical-resistant coatings known for their excellent color and gloss retention. Polyurethanes frequently are used as a topcoat with zinc-rich or epoxy primers and epoxy undercoats in coastal and highly corrosive environments. Polyurethanes cure by one of the following two methods:
These are two-component coatings consisting of an isocyanate resin and a hardener. Like epoxy, they cure from solvent evaporation and from a chemical reaction called copolymerization.
- Single component
This coating cures by solvent evaporation and by reacting with water/moisture vapor in the air. For curing to take place, relative humidity must exceed 20 percent.
- Zinc-rich primer
These hard-working primers work best when top-coated with high-performance coatings such as epoxy or polyurethane. They can be top-coated with lesser performance coatings such as alkyd or acrylic, but it would waste a good primer. To be called zinc-rich, these primers must contain a minimum of 80 percent zinc volume in the dried film.
Zinc primers protect steel in two ways: encapsulation, where the primer surrounds the steel to prevent contact with corrosive elements; and by cathodic protection, which occurs because zinc is more reactive than steel and will sacrifice (corrode) itself to protect the steel substrate. Since zinc-rich primers must be in direct and complete contact with the steel for cathodic protection to happen, surface preparation of the steel must include a minimum of commercial blast.
There are two types of zinc-rich primers: organic, which contain an organic (carbon containing) binder such as epoxy to carbon to encapsulate the zinc; and inorganic, which form a chemical reaction with steel to create a chemical-resistant matrix on the steel surface and have excellent dry heat resistance up to about 750°F.
Crissinger, CSI, CCS, CCCA, is a partner with McMillan Smith and Partners Architects, PLLC, Spartanburg, S.C. He has 22 years construction materials experience and is a construction materials specialist/product specifier.
How do they compare?
|Note: The chart shows the more common types of coatings and rates their resistance to common exposures. The listed coatings are a general type, and it is understood that some coatings within each type may perform better than those indicated, while others may perform less than those indicated. This table was prepared with the understanding that the coatings would be properly applied to properly prepared surfaces under proper conditions.|
|KEY 10: Excellent 8 to 9: Good 6 to 7: Fair 1 to 5: Poor|
|Alkyd||1. Economical |
2. Easy to use
3. Used correctly, provides reasonably good protection
|1. Low alkali resistance |
2. Limited use
|Epoxy||1. Excellent solvent and chemical resistance |
2. Good abrasion resistance
|1. Tendency to chalk and yellow|
|Latex||1. Excellent adhesion |
2. Excellent color and gloss retention
3. Easy application
4. Easy cleanup
5. Safety and VOC compliant
6. Long-term flexibility
|1. Must be kept from freezing|
|Polyurethane||1. Hard, tough, and flexible |
2. Excellent color and gloss retention
3. Good chemical resistance
4. Good abrasion resistance