Using Powder in Outdoor Applications

Q. What are your thoughts and recommendations for the use of powder on steel structures installed outdoors?

A. There have been many discussions in this column and elsewhere about the process of coating steel for corrosion protection and sunlight resistance. The most critical aspect is having a clear understanding that coating is a process that requires several steps, all performed correctly.

The factors that can cause failure start with the product design; it is more difficult to cover surfaces that are difficult to access with the powder spray gun. Avoid overlapping seams, tight creases or any similar surface that you cannot reach or see. Also, be aware that sharp edges and corners are harder to build film on; radius edges are easier to coat and protect. If you have to fasten things together, you will want to pay attention to bolt holes and fasteners to make sure that the bare steel is not exposed after the structure is installed.

You also need to pay close attention to the preparation of the surface. Steel is often treated with a series of chemicals to clean and passivate the surface and promote solid adhesion of the coating, providing some underlying corrosion protection. All the steps of the chemical process need to be completed in proper control to achieve the expected results. While chemical treatment is a very good idea for some parts, it can be harder to accomplish with some larger parts. Also, structural steel will have weld stains, mill scale and other inorganic soils that may require media blasting for removal. Media blasting without any chemical treatment is often used on larger steel structures, however. This is totally acceptable, but it must be done properly, and the lack of conversion coating means that more emphasis is needed on the powder coating layers and chemistry to ensure protection. Any blast media must be clean and free of oil and grease. Aluminum oxide or other non-ferrous media is preferred so that no steel media is left on the surface. A uniform profile should be cut into the steel that is deep enough to anchor the powder (1.5 to 2.5 mils) but not so deep that it will be hard to cover. No loose dirt or oil can be left on the surface.

Next is the selection of the powder material; an epoxy powder primer is strongly recommended. If the surface is blasted, a zinc-containing powder primer is best. This will protect against moisture penetration, and the zinc will provide a barrier of corrosion resistance. The coating has to be thick enough to bury the peaks in the blast profile. The primer is followed by one or more layers of the color topcoat. This topcoat can be a polyester chemistry for good sunlight resistance; acrylic or a fluoropolymer will add additional sunlight protection, but they will also add some cost and other challenges. If the sunlight protection must meet a certain standard, it may be necessary to use something more resistant, but a polyester powder is likely the right choice.

The powder coating has to be applied on every surface to a minimum film build as recommended by the supplier in order to protect the surface over time. It also has to be fully cured as recommended to provide full film-performance properties.

Finally, the coated structures must be handled correctly. Avoid banging the parts against one another or against hard surfaces. Do not drag them over rough surfaces. Contact with abrasives like cardboard must be avoided so that the surfaces are not marred during installation.

Follow all these steps, and you will have a great-looking structure that will last for a very long time in an outdoor environment.

Clearcoat Over Metallic Powder

Q. We recently got some parts returned that had been coated with a silver metallic powder that had turned dull and gray-looking in some areas. Our supplier tells us that we may need to apply a clearcoat over the metallic powder for this application. Any suggestions?

A. Metallic powder does not always need a clearcoat, but there are a couple reasons it should be considered, and this may be your problem. The metallic look comes from aluminum flake or mica that is mixed with the organic powder. It is possible to have the metallic aluminum flake close to the surface where it may be exposed by weathering or abrasion. If the aluminum is exposed, it can turn somewhat dull and gray, and lose the shiny look it was designed with. If the part is going to be used where it is subject to regular contact (abrasion), or exposed to sunlight and rain, it can be very helpful to add a layer of clearcoat over the metallic powder. In fact, I always recommend a clearcoat over metallic powder if it will be used outdoors. Ask you customer about the end use of any product you will be coating; if a silver metallic is going to be used outdoors you should recommend and apply a layer of clearcoat.

Comparing Different Coatings

Q. We manufacture fencing and rails for commercial and residential use. We currently do hot-dipped galvanizing, liquid paint and powder coating. How can we determine the best type of coating? How do we compare these different finishes?

A. Seems like a simple question, but it actually is not at all that simple. What is overlooked in the idea of comparing one coating type to another is the overall difference in the processes. Comparison needs to look at the substrate material, how it is prepared for coating, the use or lack of use of a primer, the specific chemistry of the coating, and the process measures in place to ensure that it is done correctly.

Take a comparison of galvanized railing versus powder coating. How will the steel rail be prepared for the powder? Will it only be blasted, only chemical treated or a combination? Will a primer be used? Will the primer be a zinc-containing epoxy or not? What is the powder topcoat material? How thick will it be applied?
Rather than compare the general technology, you need to compare a specific approach to coating. Generally, a single coat of galvanized zinc will weather better than a single coat of powder or liquid paint. A layer of powder or paint improves corrosion resistance and weathering substantially. A proper preparation technique followed by a good powder primer and topcoat will typically outperform galvanizing in terms of maintaining the original appearance and performance over time. The primer provides the corrosion protection and edge coverage and the topcoat provides the resistance to sunlight.

There are different options for both that can raise the level of performance. Of course, powder or paint can provide a lot more options in terms of appearance and color when compared to galvanized steel.