A primer on primers: The first coat is the most important

For proper results in a production environment, priming before painting is an absolute necessity. The industry likes to use the term coatings instead of paint as it is a more descriptive word for the protective nature of the substance that helps to combat corrosion and improve the surface properties.

Priming is necessary because it gives the coating something to grab onto. “Primer serves as the foundation for the chosen coating system, as well as the corrosion protection for the asset,” said Sid Oakes, global strategic account manager for Sherwin-Williams Protective & Marine Coatings (SWPM). “Primers promote adhesion of the subsequent coatings and are the heart of the corrosion protection built into coating systems designed for the asset.”

Priming is necessary because while paint is heavy in pigment, primer is saturated with binding elements.

“Primer will give you better adhesions, better quality of final finish,” said Rhoda Williams, senior brand manager for Rust-Oleum Consumer Brands Canada. “In some cases we will make custom primers for specific substrates. If you are working with steel, tile, concrete, or other surfaces, regular paints won’t stick to them, so we have special primers that will be developed and manufactured to stick to that surface.”

Not only do primers help the final coating adhere to the surface, they can solve problems on the base surface as well.

“Primers have the ability, because of their filling capabilities, to hide or fill in some defects of the metal. Industrial primers typically consist of either epoxy- or urethane-based primers depending on the overall requirements of the finished painted unit,” said Scott Laney, liquid product manager for PPG Industrial Coatings. “Primers, while not aesthetically pleasing in colour, usually aid in corrosion protection and other performance requirements of the painted unit. Paints or topcoats generally add additional protection from weathering and aid in other performance requirements, but the main purpose of the topcoat is to add colour to the painted unit since this is what drives the consumer to purchase.”

Primer needs to be matched to the surface to ensure proper coverage. There are different primers for different metals.

“Different metals have different porosities, so if you are dealing with copper, it will have a different porosity than steel,” said Williams. “You need a primer that will match the surface. Zinsser® Bondz® is specifically made to adhere to heavy and dense surfaces, and it [is effective on] about 90 to 99 percent of all metal substrates.” Putting on a proper coat of primer means that nearly any type of paint can be used to cover the surface.

For steel coatings, priming is key to preventing the formation of rust.

“Without the application of a suitable primer, the unprimed metal surface begins to corrode and rust, weakening the metal surface and ultimately leading to metal fatigue and failure,” said Laney. Many primers intended for metal applications have rust inhibitors incorporated into their formulation. These will not only prevent rust from forming, but will eliminate any rust already formed on the surface.

“A lot of our primers have rust preventive properties. They will stop any rust from adhering to the surface or will kill the rust. If you have a really rusty surface that you want to paint, even flash rust, we have a product that will remove the rust,” said Williams.

This is useful in some situations, but it is always best practice to remove the rust through grinding, cleaning, or wire brushing before priming and coating the surface. “The metal should first be properly cleaned by some chemical pretreatment process regardless of the choice of primer. This pretreatment process removes all oils and other possible contaminants from the incoming metal,” said Laney.

Many paints in the market now claim to have the primer included with the formulation of the paint. “There are a number of ‘direct to metal’ (DTM) products available in the marketplace that are formulated as barriers, and many also include inhibitive pigments in their formulation. Depending on the degree of surface preparation, the intended use and exposure, as well as the chance of damage to the coating system, DTM coatings can be viable options for asset protection,” said Oakes.

“SWPM provides an array of DTM products in several resin types, including the extremely fast-drying polyaspartic resins – EnviroLastic 940 DTM.” When used properly, primer-infused paints can lead to savings as they will eliminate some steps in the production cycle.

“PPG has developed a direct to metal 2K urethane topcoat that eliminates the requirement for primer surfaces that fall into the Class A and B surfaces,” said Laney. “This is being looked at as an overall cost savings, not so much from paint but from process whereby a paint booth and all its associated costs can be reduced or eliminated. The DTM still must possess the performance requirements of a normal primer/topcoat on these important surfaces.”

Primer can be either water- or oil-based. While there have been great advances in water-based primers, ones that are oil -still have their proponents.

“A lot of times in the commercial production field, customers are adamant that they want to use an oil-based primer. You get a different use with an oil-based primer because it is heavier,” said Williams. “When you use an oil-based primer, you do not get as much coverage out of it because water flows well and oil is thicker, but you get a thicker base that you can apply a top coating to. Oil-based primers are better to use for outdoor applications. I’m not saying that you can’t use water-based primers outdoors, but oil-based primers have more rigidity and perform better after a long time of being exposed to outside conditions.”

Curing times for primers vary, and this information must be checked. Making sure the primer has cured, and not just dried, according to the directions of the paint is important. As Williams explained, “You need to let all the solvent flash off, and if you don’t let it cure properly, then you can get off-gassing underneath your finish. If you don’t let it off-gas, you can get bubbles under your surface. The paint will trap any gas coming from the primer and you will ruin your new painted surface.”

The time needed to recoat is affected by the environmental conditions but can be hastened by baking the primed object. “Most primers typically have a minimum recommended ‘dry to recoat’ time that is well short of the actual ‘dry to full cure’ time,” said Oakes. “Inorganic zincs typically require a higher degree of cure prior to recoating, while organic zincs may be recoated once the solvents have dissipated from the film (often in as little as 30 minutes). Products like SWPM’s Kem Kromik® Universal or Steel Spec Universal alkyd primers only require a short window of dry time for recoating with alkyd or waterborne products, while a higher degree of cure is required for recoating with hot solvent coatings like epoxies or urethanes.”

While it is good practice to coat primed objects as quickly as possible, it is not always necessary. “If the baked/cured primer is subjected to the elements or can collect dirt or other contaminants, it makes good sense to scuff sand the primed unit prior to topcoating so the dirt or other contaminants are not highlighted or adversely affect the topcoat layer of paint,” said Laney.

Nestor Gula is a Toronto, Ont.-based freelance writer. He can be reached at nestor.gula@yahoo.ca.

PPG Industrial Coatings, 888-774-2001, www.ppgindustrialcoatings.com

Rust-Oleum Consumer Brands Canada, 800-387-9879, www.rustoleum.ca

Sherwin-Williams Protective & Marine Coatings, 800-524-5979, protective.sherwin-williams.com