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Four steps to pick the best site-specific coating

March 22, 2002
Four steps help you pick the right one

In any industrial plant, exposure conditions vary. Interior ceilings may be clean and dry in storage areas, yet subject to extreme humidity in manufacturing areas. Walls in laboratories may be subjected to an occasional splash or cleaning, while floors in processing areas may be frequently splashed with lubricants and scrubbed daily with hot water and harsh chemicals. Piping may be dry during one time of year while sweating profusely during another. As a result, choosing protective coatings for the wide range of structures and exposure conditions in a plant can be a challenge.

To properly choose coatings for specific plant areas, follow these steps:

  • Establish plant zones.
  • Classify corrosive environments.
  • Analyze other factors, such as application considerations.
  • Consider past performance of coating options using case histories, comparative studies and test results as predictive tools.

Establish plant zones. 

Coating selection starts with a thorough analysis of the service conditions of coated structures. To make this task more manageable, the facility and structures being protected are sectioned into zones. Maps are created to delineate zone boundaries and identify structures within a zone. 

Classify corrosive environments. 

Once zones are established, observation, historical information and testing can determine the environment’s corrosiveness. Start by identifying the material to be coated, such as carbon steel, galvanized mental, concrete, plastic or wood. For steel substrates, visual inspection of the steel surface using ASTM D610, Test Method for Evaluating Degree of Rusting on Painted  Steel Surfaces, is useful. The corrosive environment must be determined for each structure within each zone.

Environmental variables that contribute to corrosion include condensation, humidity, ultraviolet light (UV), wet/dry cycling, temperature cycling, abrasion, immersion and harsh chemicals and solvents. These variables frequently have a synergistic effect upon one another, multiplying corrosion potential. In general, corrosive environments are described as normal, light, moderate or severe. 

To assist in classifying corrosive environments and choosing the protective coating system to use in a particular plant area, the Steel Structures Painting Council (SSPC) developed Environmental Zones Painting Systems, a program that identifies 12 environmental zones, ranging from 0 (interior exposure) to 3E (severe chemical exposure). Along with a description of each zone, it recommends coating system suitable for use in each area. 

Analyze other factors such as application considerations.

Other considerations are involved in specifying coatings. Some questions to answer include: Can the surface be properly prepared to accept the recommended coating system? Will production continue while painting is in progress? Will plant workers be present in adjacent areas? Will plant safety be influenced? What physical limitations may impede coating application?

To help plant specifiers evaluate these variables, industrial painting contractors and coatings manufacturers may be called on to make recommendations. Many of the newer coatings require less labor to apply minimize disruption to the plant and shorten downtime. 

Consider case histories, comparative studies and test results.

Predicting how a particular coating will perform under site-specific conditions is extremely important. Case histories and comparative studies aid in this process. Results from accelerated laboratory exposure tests are also useful, although not all accelerated tests provide meaningful information. The long-standing salt-fog method (ASTM B117) has been found to be particularly poor at forecasting coating performance. While no single exposure test can duplicate actual atmospheric conditions exactly, the newly adopted cyclic corrosion test (ASTM D5894) offers an improved technique for comparing the relative durability of protective coatings. 

To aid the decision-making process, here is a sample of coating systems used in a broad range of plant environments.

Pulp and paper

At a large paper plant, mill engineers determined a complete repaint was needed of the interior and exterior of a steel-framed, 100 ft.-high exhaust stack and  precipitator structure. Years of exposure to the elements had left the stack and precipitator rusted and bare. Because abrasive blasting was not allowed, a protective coating system, which could be applied over the existing surface, was needed. Other challenges included the structure’s heights, the 120ºF temperature of the stack’s skin and the fact spray paint couldn’t be used. As a result, products that could be easily brushed and rolled were required. 

Workers applied a two-coat system of a high solids, high build, fast drying polyamide epoxy primer and a high solids polyurethane coating at 4 to 6 mils DFT and 3 to 4 mils DFT, respectively. The primer is a two-part epoxy that can be brushed and rolled over a marginally prepared steel surface. The two-part polyurethane topcoat provides a high-gloss, UV-resistant finish. The precipitator stack received two coats of a surface-tolerant epoxy mastic aluminum II. This two-part epoxy can be applied to surfaces as hot as 120F. The product was applied using a brush and roller to achieve 3 to 5 mils DFT per coat. 

Another of the company’s plants, the exterior of a carbon steel caustic soda storage tank had significant rusting. A three-coat moisture-cured urethane coating system was chosen to allow for quick application and recoating ability. The intermediate coat was a single-component, moisture-curing urethane with micaceous iron oxide, which outperforms epoxy mastics when overcoating old red lead coatings. The topcoat was a single-component, moisture-curing urethane designed for low-temperature or high-humidity applications and provides excellent UV and chemical resistance. 

Food and beverage

Coatings in direct or incidental food contact must comply with strict FDA regulations. Further, they must withstand the harshest conditions — from frequent cleanings with caustic disinfectants to high humidity, steam and condensation, as well as contact with oils, dirt, grease, blood, alcohol and various foodstuffs. Complicating matters, there are a wide range of exposure conditions in food and beverage facilities. These include dry storage rooms, cold rooms, canning and slaughtering areas.

At one of the nation’s largest bakery facilities, several coatings have proven successful. A two-component waterbased, catalyzed epoxy resin coating is frequently used for equipment and equipment framework. A two-part, epoxy-polyamide coating has performed well on structural steel and in non-food contact areas, such as walkways and stairways. When non-skid floor coating is required, a catalyzed epoxy floor coating has been used with good results. The coating dries rapidly to a tough, high-gloss finish, and meets USDA standards for protective coatings not in direct food contact. For cold rooms and steam pipes where humidity is high, the plant uses moisture-cure urethane coatings. They actually rely on atmospheric and surface moisture to form solid paint films.

General manufacturing

For manufacturing facilities, a key consideration is lighting, both in manufacturing and warehouse areas. Ceiling paints with high light reflectance can brighten a plant, improving visibility without increasing energy costs. “Dryfall” coatings are the most commonly used because their overspray dries to a powdery substance before reaching the ground.

Do’s and don’ts for specifying coatings

  • Identify the substrate type to be coated (carbon steel, concrete, galvanized metal, plastic, wood, previously painted). 
  • Determine the corrosiveness of the environment. 
  • Identify surface preparation needs and impediments. 
  • Analyze the application situation.  Consider the cost of downtime, adjacent structures, workers in nearby areas, and other relevant factors.
  • Ask for paint manufacturers’ recommendations for coatings that minimize labor and reduce downtime.
  • Don’t specify coatings without benefit of case histories, comparative studies or testing results.
  • Don’t specify coatings based on cost-per-gallon without considering coating application and performance advantages.

Two types of dryfall coatings are available: standard-grade dryfalls, which provide two-coat coverage, and premium-grade dryfalls, which require only half the amount of material to achieve the same degree of hiding as their older counterparts. These coatings are frequently specified for steel framework, joists, sprinkler pipes, duct work and conduit. 

A number of other coatings are used in light industrial and general manufacturing environments. Direct-to-metal acrylic primer/finish coatings are 100-percent acrylic emulsion, waterborne and corrosion resistant. They are used for metal surfaces, such as stairs, ladders and handrails, or topcoated with an all-purpose alkyd enamel for use with a wide range of equipment. For interior industrial surfaces, other topcoats include water-based catalyzed epoxies and two-package, epoxy polyamide coatings.

Petrochemical and chemical

To protect against chemical attack and corrosion, the petrochemical and chemical industries frequently use moisture-cure urethanes; self-priming, surface tolerant epoxies, which bond to marginally prepared substrates; and ultra high-solids (98 percent) amine epoxy tank linings, which permit the creation of an 18-to 22-mil lining in one spray coat. Fiberglass reinforced plastic linings are appropriate in many instances for reinforcing chemical and petrochemical tank bottoms. 

One chemical plant tested a dozen coating systems for 13 months and found that the most durable and best-suited system for its plant environment was a two-component, polyamide epoxy, zinc-rich organic primer combined with a direct-to-metal, waterborne acrylic topcoat. The organic zinc rich primer protects steel by providing a moisture barrier and has the ability to resume protection if damaged. The topcoat provides excellent corrosion, chemical resistance, and color and gloss retention. Wind carry testing at the plant showed that the overspray for both coatings quickly turned to a dry dust, with no trace at 125 feet away.


The pharmaceutical industry is highly regulated by the FDA. A crack in a floor or a flaking coating system can shut down production, sapping the bottom line of millions of dollars. These facilities often require seamless floors and a smooth floor-to-wall envelope, so that microorganisms have no place to hide. These coatings must last when subjected to frequent, rigorous cleaning. For these plants, consider water-based, catalyzed epoxy coatings, flexible epoxy crack isolation systems and epoxy-terrazzo flooring, which tenaciously withstands abuse. 

Dave Schutz is a  product information specialist with Sherwin-Williams Company. He can be reached at 216-566-5277 or [email protected].

Photos: Sherwin-Williams Co.

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