Comparing and testing generic topcoats for exterior use

One of the important factors a buyer considers during the paint buying process is how well topcoats perform during their service.

Also known as weathering finishes, these coatings are designed for ultraviolet (UV) exposure during exterior use. They’re common in any industry that requires equipment or structures to be used or kept outdoors. Topcoats are the first line of defense against the elements, providing protection to intermediate coats which are designed to protect primers.

And it’s important to note right away that there’s no such thing as a coating that doesn’t weather. All coatings fade and break down eventually. With a greater understanding of topcoat formulations and quality testing methods, you’re empowered to make the choice that offers the best protection for your critical assets.

Accelerated weathering testing

In these tests, UV and moisture exposure is simulated in controlled settings over abbreviated time frames to judge how well coatings stand up to the elements. This is a preferred method for testing and comparing weathering finishes because it’s impractical to rely on real-time field tests that would take years to complete.

However, accelerated weathering testing is an imperfect method that requires strict controls and an understanding that actual results may vary in the field. Consider these practical issues:

• The UV lamps used to simulate sunlight in these tests emit radiation only in a very narrow bandwidth compared to the broad-spectrum UV radiation produced by the sun.
• Proper testing in controlled conditions cannot replicate or predict the diversity of ever-changing operating conditions in the field.
• There’s no accurate way to translate results from accelerated weathering testing to actual useful service lives for coatings in the field. The testing should only be used to compare coatings against one another in similar conditions.

Comparing coating formulations

Tests conducted on weathering finishes are designed to shed light on a few different things, such as:

• Resin quality – Resins are key components in coatings because they form the matrices that hold color pigments in place. When testing coatings to judge the quality of their resins, it’s best to use white or light-colored formulas that lack color pigments. That’s because differing color pigments degrade at differing rates, and that could skew the results of tests targeting resins.
• Color fastness – Not all pigments are created equal—even like-colored pigments. For example, the red pigments used to color a muscle car are of far higher quality than those used in other, shorter-term applications. When testing for color fastness, choose formulations with similar resin quality so that resin degradation doesn’t interfere with pigments.
• Gloss retention – When comparing the rate at which gloss loss occurs in a coating, it’s critical that the angles to which lamps are set remain constant. Differing angles alter the way light impacts a painted surface, and results will not be reliable.

It’s important to maintain other constants in tests, too, like film thickness, application method, curing conditions, testing exposure cycles and testing surface preparation. Ignoring experimental controls allows too many variables to interfere with test results.

Performance benchmarks

The Society for Protective Coatings’ (SSPC) standard for two-component weatherable aliphatic polyurethane coatings (SSPC Paint 36) defines three performance levels for weathering finishes.

Level 1 finishes are rated based on 500-hour exposures during accelerated weathering tests. Level 2 finishes are rated based on 1,000-hour tests. Level 3 finishes are rated based on 2,000-hour tests.

Alkyds or oil-based coatings are typically tested in the 250 – 1,000 hour range; most of these coatings sustain significant gloss loss at the high end of this range.

Epoxies lose gloss quickly and begin to chalk after a couple hundred hours of exposure.

Acrylic formulations can vary widely, but they typically hold gloss for between 1,000 and 2,000 hours.

Polyurethanes are even more variable in formulation than acrylics and are commonly tested for several thousand hours. Polyurethane clear coats care often tested into the 5,000 hour range.

Siloxanes and fluorourethanes known as “ultra-weatherables” have shown impressive gloss retention even after 9,000 hours of accelerated weathering testing.

Comparing US Coatings finishes

US Coatings offers many weathering finishes. Buyers need to carefully consider all aspects of an asset or structure prior to choosing a weathering finish, including:

• The intended use of the asset and whether such a finish is even appropriate.
• Whether the aesthetic appeal of an asset is important to its end use.
• Whether a such a finish is being used in conjunction with other coatings or asset protection systems.
• How frequently you expect to re-coat the asset.

Use the chart below to determine which US Coatings product is the best match for your exposed assets:

We’re here to help you answer any questions you may have. If you want to talk through your options further, let’s talk. You can also explore our products in greater detail by downloading our product data sheets here.

Choosing the right weathering finish is just one step in assuring your next painting project goes off smoothly. In this guide, we take you through all the elements of a productive —and painless— industrial painting program.

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