Coating San Francisco’s Golden Gate Bridge

TUESDAY, FEBRUARY 22, 2022


The Golden Gate Bridge in San Francisco is known for its unique orange color, which requires continuous coating work. Its span reaches 4,200 feet, and its constant exposure to salt water from the Pacific Ocean requires the painting of the structure and maintenance from rust and corrosion be an ongoing task.

Construction of the Golden Gate Bridge began in 1933 and was completed in 1937, connecting San Francisco and Marin County, California. The bridge opened to traffic on May 28, 1937.

Since its completion, the steel structure has also undergone major repairs and improvements, including an earthquake retrofit, replacement of the bridge deck in the 1980s and safety railing installations along the sidewalks. The Golden Gate Bridge Highway and Transportation District estimates that about 40 million vehicles travel on the bridge per year.

The original plans submitted by Chief Engineer, Joseph B. Strauss, called for a hybrid cantilever and suspension structure across the Golden Gate, with a suspension bridge of this length never having been done before.

Coating the Bridge, History

The Golden Gate Bridge was not named for its color, but refers to the entrance to the San Francisco Bay from the Pacific Ocean, the Golden Gate Strait.

During its design phase, some project designers suggested a carbon black and steel gray palette, to match the Bay Bridge, while the U.S. Navy proposed black and yellow stripes to ensure greater visibility for passing ships.

The bridge’s orange color, named “International Orange,” was selected by consulting architect Irving F. Morrow because its warm color blends well with the span’s natural land setting and stands out against the cooler colors of the sky and sea. It also provides enhanced visibility in fog for passing ships.

“The effect of International Orange is as highly pleasing as it is unusual in the realm of engineering,” wrote Morrow in a report regarding color and lighting.

Additionally, the red-orange color was chosen with corrosion in mind. With its exposure to salt water and fog, International Orange will hide rust better than a gray. To first coat the bridge, about 110,000 gallons of paint were used.

In one of its first annual fiscal reports, the largest item in the maintenance budget was for the painting of the structural steel. “The past fiscal year, the sum of $112,431.84 was expended for painting of the Bridge structures,” listed the report. “This amount was divided as follows: Labor, $86,589.13; brushes, tools, etc., $5,430.67; paint, $20,412.64.”

The report also called for an acceleration of the original painting program, noting the severity of exposure and the failure to use special treatment, including sandblasting or flame cleaning, prior to the erection of the steel surfaces. A majority of the original paint failures was reportedly caused by the loosening of mill scale.

“The exposure to salt-laden fog is more severe at the Golden Gate than any other bridge in the Bay Area. Not only is the fog extremely active in attacking the paint film but it also limits the hours when painting can be done,” it continued.

It is reported that the recent annual budget for paint, including the sand used to blast off the old coat, has been about $300,000 per year. In 2021, the Golden Gate Bridge Highway and Transportation District used 720,000 pounds of sand and 2,590 gallons of paint.

According to the District, purchasing paint is done through a competitive bidding process, with paint currently being supplied by The Sherwin-Williams Company. While the paint is made to match the International Orange color formula, the closest off-the-shelf paint color customers can buy is “Fireweed.” The paint can also be mixed in stores with the color formulation on the district’s website.

Originally, contractors used a lead primer with a lead-based topcoat. This method was replaced by 1968 due to aggressive corrosion, in favor of an inorganic zinc silicate primer and vinyl topcoats. In 1990, the topcoat was reportedly changed to an acrylic emulsion to meet state requirements regarding volatile organic compounds (VOCs).

When the two main cables and the north approach viaduct structures were repainted a decade ago, painters used a system of a primer and then two coats of a finish, which proved to be short-lived.

“The district found that they were getting some discolor — white rust coming through that system. The exposure with the fog and salt layer is a very difficult atmospheric climate cycle,” said Derrick Castle, the business development manager for bridges and highways with Sherwin-Williams.

“We did some investigative work with the district and found that the white salting was a reaction to the salt in the air and some zinc primer that came through the acrylic.”

In response, the District collaborated with Sherwin-Williams to develop a new three-pronged system that starts with a primer, adds an epoxy intermediate and is topped with an acrylic finish coat. Products used include the company’s Corothane I GalvaPac 1K moisture-curing urethane, Macropoxy 646 Flake-Filled and Sher-Cryl 1300.

“We put on two coats of the finish,” said Fred Mixon, Paint Superintendent for the GG Highway and Transportation District. “We use that because it’s flexible and holds up to the UV rays and protects from salt. The epoxy doesn’t hold up well against UV, so that’s why we put the acrylics on top.”

The paint, which is delivered in five-gallon buckets, is ordered for each project specification.

SFGate reports that Mixon’s team is currently focused on repainting the south approach section which began about three years ago. When asked about a local story that crews paint the bridge from end-to-end each year over and over again, he replied, “It is a myth.”

“Our inspection team inspects areas on the bridge and then numbers them from high to low priority,” Morrow said. “If there is a section that is high, we jump on it right away.”

Mixon oversees a crew of 42, including inspectors, ironworkers, electricians and painters. These crews have to battle wind as they are typically suspended high above the bridge to inspect and repair corroding steel, remove plates and bars to provide access and set up 70-by-70 foot containment areas for blasting.

“We’re at 100% humidity all the time due to the fog, and we can go through three seasons in one day on the bridge,” Mixon said. “We have to have at least 85% humidity so we can apply the finish.”

To combat this, electricians run generators to power the equipment, lights and humidifiers. Painting crews are separated into four groups, with two crews sandblasting and two painting crews behind them.

“We put forth a good effort because you can’t cheat the steel,” Mixon said. “Every time I touch that paintbrush, I can’t help but to have pride — it’s the Golden Gate Bridge! The most iconic bridge in the world.”

   

Tagged categories: Bridges; Bridges; Coating Application; Coatings; Corrosion; Corrosion protection; Historic Structures; Industrial coatings; NA; North America; Paint; Program/Project Management; Sherwin-Williams; Steel

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