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One Coat, Two Coat, Red Coat, Blue Coat

Posted on July 28th, 2016

When a customer asks me about two coat powder coating applications, I usually reply “I can coat it as many time as you want!” What they are referring to of course is a primer and topcoat system, which is common with wet paint. There are a few things you want to understand before your next project. Most powder manufacturers offer their own primer/topcoat systems for improved corrosion resistance. Though these systems are most often used on iron products, some are used on aluminum as well. Three more common primers are zinc rich primer, zinc free epoxy primer, and epoxy e-coating. The question is, how do they differ, how much do they cost, and which one is right for the job?

The first two coat system I will discuss is the e-coating/powder coating systems. E-coating, which is also known as electro-coating or electro deposition coating, is basically plating metal parts with paint. It is almost always a dip process and is usually a requirement in the automotive industry. The process involves multiple stages of cleaning rinsing and sealing, a conditioning preparation stage, the electrically charged paint tank, and a series of rinses to remove excess paint. The pre-treatment is usually zinc phosphate with a chrome sealer, (which is arguably one of the highest performance processes in mass pretreatment for iron parts). Rinses are generally de-ionized (DI) water or at least reverse-osmosis (R/O) filtered water. The pre-treatment can be dip only or a combination of dip and spray. The zinc phosphate and seal operations provide an excellent layer of corrosion resistance on their own, and are enhanced by the application of the paint.

E-coat paint is a conductive paint that requires heat to set up and cure. The paint is usually black though it can be other colors, and epoxy is the most common resin, though acrylic is also available.  It is applied in a tank that has either a cathodic or anodic charging system. The liquid in the tank is charged and a grounded part is run through the paint. The amount of charge determines the amount of paint applied to the part (usually .5 to .75 mils of paint), similar to the electrostatic charge of a powder gun in relation to the film build of the powder. Once the part leaves the paint tank, the excess paint is removed by dip rinse, spray rinse, or a combination of the two. The part them goes into an oven to cure the paint.

Once the part is finished with cured e-coat paint, it can be top coated with any standard powder coating. Because the majority of e-coat paint is black epoxy, it provides the superior corrosion protection.  The powder coating topcoat provides the color and UV stability. The combination is a durable, weatherable finish with outstanding performance.  Some companies have finishing lines that have both e-coating and powder coating capabilities on a single line. This allows them to be very competitive, and eliminates extra freight and handling. The cost of the e-coat is generally very competitive, and there is no special procedure for applying the powder top coat. If the two coats are applied on different lines or at different facilities, however, the cost for this system may be considerably higher and may be cost prohibitive. Some products may be too large or too geometrically complex to e-coat properly, but most castings, stampings and fabrications can take advantage of this process.

The other two-coat system I want to discuss is the powder primer, powder topcoat system. There are two common types of powder primer currently available. Though both are generally epoxy based, one is zinc rich and the other is zinc free. Though the data sheets on both are often identical, there is a case to be made for both. The fact that they are generally epoxy based stems from the fact that epoxy resins have better corrosion resistance propertied than polyester resins. Most powder companies sell zinc rich primers for use with ferrous substrates, and zinc free primers for non-ferrous applications.

Zinc rich primers suggest that the presence of zinc provides corrosion protection on iron in a similar fashion as zinc phosphate and galvanizing do. That is, that there is a sacrificial property of the zinc to protect the iron underneath the coating. When dealing with aluminum, stainless steel or other iron free base metals, the epoxy-based primer offers similar protection to the epoxy e-coating, though without the pre-treatment protection. Generally, the recommended pre-treatment for powder primers is abrasive blasting to clean the surface and provide an aggressive surface profile for the powder to adhere to. The thickness of the primers is generally the same as standard powder applications, 2-3 mils.

Once the primer is applied, it is partially or fully cured, depending on the manufacturers recommendations. It is not recommended to apply powder topcoat over powder primer in the same application (i.e. dry on dry). The primers are usually grey in color, so the top coat provides the final color. If the product is to be used in an outdoor environment, the polyester topcoat will provide the UV stability as, while the epoxy primer provides the enhanced corrosion protection. The final dry film thickness of the coating will be around 5 to 7 mils. The cost for the powder primer/powder topcoat system is usually slightly less than twice the cost of a single coat system. This is because the part is hung only once, and no cleaning is required between coats, even though it must be coated and cured twice. Most castings, stampings and fabrications can be finished with a powder/powder system, and the performance of the finish and the flexibility of the application method allow a wide variety of product sizes and geometries.

There are, of course, other ways to achieve a two coat finish with powder, such as powder coating over galvanized metal and powder coating over a wet primer. There are also powder primers that are applied prior to a wet topcoat, mostly in military applications, as with CARC coated products. There are also many products that have a standard polyester powder base coat with a clear topcoat to improve wear resistance or gloss appearance. In any event, if you want to enhance the performance of you product, consider using two coats, because two coats are better than one!

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