Frederick Douglass Bridge: Two-Coat Shop Throughput Gets a Boost

In the quest for productivity, coatings application shops often ask what the best system is to meet their project’s deadline. Can they accelerate topcoating times? Can they reduce the number of coats needed? Can they get coated steel out of the shop one to two days faster? Importantly, they also want to know: Will the coating system maintain the desired level of performance for the end customer? Following recent shop application studies on two-coat systems for steel bridges featuring organic zinc-rich primers, the answer to all these questions is a resounding “yes.”

Such questions are certain to arise when a shop is tasked with coating more than 700 tons of structural steel for a major bridge project, such as what was required for the new Frederick Douglass Memorial Bridge in Washington, D.C., which opened in September 2021. The project team investigated multiple solutions before arriving at applying a twocoat system to all the structural steel for the bridge. By selecting an organic zinc primer with this system, the application shop saved up to two days of time when coating steel members, enabling it to accelerate throughput and keep the project ahead of schedule.

The choice of the two-coat, organic zinc/polysiloxane system for this project was based on National Transportation Product Evaluation Program (NTPEP) testing data secured by Sherwin-Williams Protective & Marine. The company had its Zinc Clad® 4100 organic zinc and Zinc Clad II Plus inorganic zinc primers tested in combination with its Sher-Loxane® 800 polysiloxane as the topcoat for a two-coat system. Results demonstrated that either combination of the two-coat system would offer comparable longevity and performance in the field compared to that offered by traditional three-coat systems featuring inorganic zinc primers, intermediate epoxy coats and polyurethane topcoats.

With durability confirmed for a twocoat system, the fabricator, Veritas Steel, was able to drop an entire application step — saving one-third of the coating application labor and at least two to six hours of waiting time per piece of steel by eliminating the intermediate coat. Further enhancing efficiencies, Veritas Steel opted to use the organic zinc primer, which, compared to the inorganic zinc, saved the applicator up to 23 hours of waiting time before either applying touchups or the topcoat. Because the NTPEP testing showed no performance difference between the organic and inorganic zincs, the Frederick Douglass Memorial Bridge is expected to retain its corrosion protection for at least 25 to 30 years. With its polysiloxane topcoat, the bridge is also expected to maintain its white color and glossy appearance for just as long.

Comparable Performance Between Two-Coat and Three-Coat Systems

Traditional coating specifications for bridge steel typically recommend using inorganic zinc primers as part of a threecoat system due to their perceived premium corrosion protection. However, the new NTPEP data demonstrates that two-coat organic zinc systems can deliver the same or better corrosion protection. For example, following ASTM D5894 cyclical weathering testing, two- and three-coat systems featuring either inorganic zincs or newer organic zincs did not exceed the 2 mm average corrosion undercutting limit required to be approved for use by bridge owners. Given these comparable results, moving to a two-coat system is now an easier decision for application shops focused on boosting throughput.

A traditional three-coat system for bridge steel includes an inorganic zincrich primer that sacrifices itself to fight corrosion, an intermediate epoxy coat that builds film thickness to protect the primer coat and a polyurethane finish coat. The topcoat resists degradation to protect the lower layers while also providing color and gloss for aesthetics.

The two-coat system used for the Frederick Douglass Memorial Bridge featured an organic zinc primer and a polysiloxane topcoat that combines the hardness of an epoxy and the flexibility of a polyurethane. The system could have also used an inorganic zinc primer with similar field performance results. However, the significant waiting time differences between the two primers sealed the decision to specify the organic zinc.

Organic Zincs Primed for Better Productivity

The primary difference between inorganic and organic zinc primers lies in the productivity gains offered by the significantly faster combined recoat and dry-to-handle times for organic zincs. Table 1 compares these times between two- and three-coat systems using either primer. The times represent how long applicators need to wait before the steel can be coated with the next layer or moved out of the shop and loaded for delivery. The chart includes two potential application steps per coat to account for touchup needs following inspections, as any coatings that need to be repaired will necessitate the same wait time before moving to the next step. If touchups aren’t needed, those steps can be eliminated from the total completion time.

The most notable timing differences among the system options is between the primers, with inorganic zincs requiring a wait time of 18 to 24 hours before the next application compared to just one to three hours for organic zincs. This 15- to 23-hour savings is significant in terms of shop throughput, enabling applicators to apply the polysiloxane topcoat from the two-coat organic zinc system that much sooner and accelerate the coating completion schedule.

Following the inorganic zinc primer application for the three-coat system, applicators will need to wait 18 to 24 hours before either repairing any deficient areas or applying the intermediate epoxy coat. That epoxy coat will require a four- to eight-hour wait time before either being retouched or applying the topcoat. Next, the polyurethane finish coat will require a two- to four-hour wait before retouching the coatings or moving the completed asset out of the shop.

For a two-coat system using an inorganic zinc primer, the first step is identical to the three-coat system. Efficiencies are gained with the elimination of an entire coating layer, plus the shorter recoat and handling wait time of just four to six hours for that system’s polysiloxane topcoat compared to the total combined six- to 12-hour wait required by the intermediate and finish coats in the three-coat system (not including potential touchup rounds in either case, which would double the timing).

As noted, the difference in recoating and handling times between inorganic and organic zinc primers provides the greatest efficiency in the shop throughput equation. That’s primarily due to the 15- to 23-hour time savings realized with one coat of an organic zinc primer. But it also relates to the predictability of that curing time. Organic primers cure reliably within one to three hours, helping applicators better plan production, inspection and delivery schedules. Following the consistently short wait, applicators can topcoat the steel with a polysiloxane and wait the four to six hours required before touching up or moving the steel.

Actual coating times naturally depend on the application itself, the environmental conditions and how many shifts a shop runs. But, in general, with a three-coat system, a piece of steel will take about two to three days to move through a shop before being able to be loaded for delivery to a job site. Using a two-coat organic zinc/polysiloxane system instead cuts at least one full day out of the work, representing major productivity gains and cost savings.

Polyaspartics Offer Topcoat Alternative

While the timing indicated in Table 1 covers the use of a polysiloxane topcoat in the two-coat system, a polyaspartic topcoat would offer a similar recoat and handling time of about three hours for comparable total application times. The primary difference between the coatings is that polysiloxanes provide longer gloss and color retention for enhanced aesthetics. In addition, shop applicators would be limited to only using the faster-curing organic primer, as polyaspartics are not recommended for application over inorganic zincs. But that’s a welcome limitation due to the faster throughput the organic zinc allows.

A New Approach for Coating Bridge Steel

While historical practices would suggest that owners and engineers specify three-coat inorganic zinc/epoxy/polyurethane systems for coating new bridge steel, the recent NTPEP testing of two-coat inorganic zinc/polysiloxane and organic zinc/polysiloxane systems proves these reduced-coat systems can perform just as well in the field. There’s no difference in their performance level or corrosion resistance because the polysiloxane topcoat combines the properties of film build, aesthetics and protection — in a single application — that the three-coat system’s epoxy and polyurethane layers provide. This revelation has opened the door for asset owners, engineers and shops alike to save significant labor and costs by eliminating an application step from the process.

Furthering efficiencies, these parties can opt to use organic zinc primers in two-coat systems. The primers offer highly predictable cure times and combined recoat and handling times that are up to 23 hours faster than those for inorganic zincs. This significant savings — which could reach 46 hours if primer coat touchups are needed — enables shops to drastically boost productivity and throughput by one to two days to meet customers’ demanding coating schedules.

Figure 1. Using an organic zinc primer in a two-coat system for coating bridge steel accelerates topcoating times to increase shop throughput for more productive operations.

Figure 2. An intermediate coat of Dura-Plate® 235 sprayed onto all the sludge trough surfaces sealed up the concrete to prevent water infiltration and protect the substrate from wear.

Figure 3. A deep blue topcoat application of Poly-Cote™ 115 in the sludge troughs allows for expansion and contraction due to temperature fluctuations.