Durable Powder Coating for Extreme Weather

Weather coating provides durability even in Arizona’s flood plains

An empty road running toward rocky brown mountains is lined by scrubby bushes and green bollards.
Arizona’s flood plains feature a climate that quickly rusts exposed iron alloys.

Large-scale outdoor installations require significant investment; to be cost effective, each piece of the installation must stand up to long-term weathering. Under-performing products can result in massive, unexpected financial losses. Selecting durable products is challenging in regions with extreme weather, because online descriptions aren’t always backed up with rigorous real-world testing. How can purchasers predict which weather coating can stand up to years of wear?

Building for an extreme climate

In order to create a long-lasting installation, it is essential to select a site-appropriate finish. Selecting a high-quality product isn’t always enough to guarantee long-term performance if it has been tested in very different climate conditions. It pays to consult an expert to determine which coating type and formulation chemistry will best meet aesthetic, function, and service life requirements in a given environment.

The problem of long-term wear in extreme site conditions was a concern for the tribal council of the Pascua Yaqui Indian Reservation. They needed to install a transportation management system around the Casino del Sol, including dozens of cast iron bollards, which must maintain their appearance and structural integrity over a long service life.

Most professionally-produced bollards will stand up to conventional North American weather. The Yaqui council, however, was dealing with some unique environmental challenges.

Casino del Sol is located about 15 miles southwest of Tucson, in an area commonly referred to as the Arizona flood plains. The local climate is extreme: between July and September, the region experiences intense rain that locals refer to as “monsoon season.” It also has some of the hottest weather in the country, with an average daily high of 101 °F (38 °C) in July. The weather is not always warm, however. In December, lows can get down to 28°F (-2.5°C), meaning that outdoor installations need to cope with cold weathering as well.

Ductile iron is a good choice for a material to manage the thermal expansion and contraction. However, any iron product rusts, and these conditions are the sort to guarantee accelerated corrosion. The Yaqui council needed a finish that would completely insulate the metal surface from the elements throughout the year’s weather cycle.

It was immediately clear that a generic surface finish wouldn’t cut it. A finish needs to be resistant to weather conditions that are normal for the project location—even if that “normal” weather consists of blazing desert sun and annual flooding.

Surface finish selection

When selecting a site-appropriate surface finish, specifiers need to balance initial cost against maintenance and service life requirements. A cheap finish will require more frequent upkeep and have a shorter service life, so a higher-quality surface finish is usually less expensive over the long term. In the harsh climate of Casino del Sol, maintenance and replacement costs for dozens of bollards with an inadequate coating would add up fast.

Powder coating provides a thick, uniform coating with superior corrosion resistance

Bollard coatings are usually available in either wet paint or powder coating. Both coating types contain resins, additives, and pigments. The major distinction is that wet paint contains solvents, but powder coating contains none. Because of its natural durability, powder coating is the more popular option among bollard suppliers.

The Arizona flood plains are an unforgiving environment for wet paint. Moisture, sunlight, and heat are each “damage functions” that can lead to the breakdown of resin in painted surfaces. When all three damage functions are combined, the effect is much more severe. In such an environment, even premium paint with a high epoxy resin content would require excessive maintenance.

A powder-coating finish was selected for the site. Powder coats are applied through electrical charge, providing a coating that is contiguous, without even pin-hole cracks. Electrostatic application allows for a thick, uniform coating that provides superior corrosion resistance. Powder coating is also much less vulnerable to cracking and peeling than wet paint because it is thermally bonded to metal during curing.

A man in coveralls and a mask shoots a stream of powder from a gun at a hanging metal item
Powder coating’s durability comes from electrostatic application and thermal bonding.

Of course, selecting a finish isn’t as simple as wet paint versus powder coating. The various resins used in commercial powder coating – epoxy, hybrid, urethane and polyester – all have relative strengths and weaknesses. Different resins are used depending on the performance criteria.

Powder coating durability and weather-resistant resins

To weather monsoon season, the bollards at Casino del Sol would require a finish with exceptional resistance to moisture, UV exposure, corrosion, and heat. No single resin has optimal resistance to all types of damage – a multi-coat system would be required to effectively protect the bollard’s cast iron surface from all the damage functions common to Tucson.

The bollards were finished with a powder coating system consisting of an epoxy primer and a green semi-gloss polyester top coat.

Epoxy coatings have excellent weather, corrosion, chemical, and abrasion resistances, however they have one major weakness: poor chalk resistance. Chalking occurs when UV exposure breaks down the coating surface, causing resin and pigment particles to lose adhesion and form a whitish, chalk-like layer. Over time, chalking erodes the paint layer and decreases surface protection. In Arizona’s blazing summer sunshine, an uncovered epoxy coat would chalk rapidly.

Unlike epoxy coatings, polyester-based powder coatings have excellent chalk resistance. Their outstanding adhesion, corrosion resistance, and ability to withstand heavy sun exposure make polyester powder coatings an effective top coat for outdoor applications.

When combined, a powder-coated epoxy primer and polyester top coat create a highly durable finish that can withstand intense UV radiation, rain, wind, and any other variation of standard weather.

Even with a durable exterior powder coating, monsoon season and flood waters remained a major concern for the bollards’ long term structural integrity. To address this concern, an additional layer of moisture-resistant epoxy primer was added to the interior of each bollard to prevent damage from long term exposure to standing water.

A closeup of a green bollard shows little wear. Scrub bushes and a blue sky frame the installation.
5 years after installation, these bollards are in almost-new condition.

Standing strong

After more than 5 years, the bollards are still in service and showing minimal wear. Thanks to site-appropriate design, the bollards are withstanding everything the Arizona Flood Plains throws at them.

In the meantime, materials science has been advancing. Today, the same internal protection would be needed, but the tribal council could also choose IronArmor for the outer shell. Consult with us about your application to see which is most appropriate to project budget and local climate conditions.