Primer and Undercoat

For even the most experienced professional, using the words “primer” and “undercoat” interchangeably can paint a picture of confusion. One basic rule of thumb to remember: An undercoat is always a primer, but a primer is not always an undercoat.

Undercoats are a subset of the primer category and serve a special function, says Andrew Kinnen, Product Manager of Architectural Coatings for The Sherwin-Williams Co., in Cleveland, Ohio. “An undercoat’s key purpose is to provide a smooth, uniform, even surface for topcoats. They are particularly useful with enamel topcoats, because they supplement topcoat film thickness and help impart a more substantial, denser finished look.” Undercoats are used primarily on wood substrates, adds Kinnen.

Undercoats usually have some degree of build to cover surface imperfections and are often sanded before recoating, says Hugh Champeny, Corporate Manager of Product Development of Kelly Moore Paint Co. in San Carlos, Calif. “The term ‘primer’ is commonly used in the paint industry to refer to a base coat or a first coat used on exterior surfaces,” he says.

Good primers seal, hide and bond to form a firm foundation for the topcoat. Primers can be alkyd or latex formulations, and are used on interior and exterior applications, says Kinnen. Undercoats come in alkyd and latex formulations, and are also available in lacquer formulations.

Alkyd-based paint releases a stronger odor and takes longer to dry than latex-based paint, according to contractor Dave Martin, owner of Dave’s Painting in Petaluma, Calif. “The advantage is, it stays wet longer, allowing it to penetrate the surface and bond deeper. The latex dries quicker, and this allows you to paint faster. But latex may not block tannin stains on the first coat. Oil is a better blocker, but oil-based paints are being phased out.”

Generally, primers bond well to flat or low-gloss surfaces. “Glossy surfaces or surfaces that have low surface energy are more difficult to bond to,” notes Richard Hardy, President of XIM Products, Inc., in Westlake, Ohio. Hardy says all primers must be formulated with select additives that promote surface wetting specific to the surface and bonding to a specific surface.

A specific category of primers — bonding primers — are formulated to tightly anchor topcoats to slick surfaces that are difficult to paint, says Kinnen. “These surfaces can include polyvinyl chloride pipe, laminates, galvanized metal, hardboard, plastic trim/molding, varnish/enamel and gloss surfaces,” says Kinnen, adding that the product’s unique raw materials give it maximum adhesion. The high percentage of resin or binder in undercoaters gives them excellent adhesion to hard, dense surfaces, adds Champeny.

Not One and the Same
“My rule of thumb,” suggests Martin, “is if it is a new surface, use a primer. If your painting an existing surface that has been painted before, use an undercoater.”

Various needs call for various approaches, notes Hardy. “When the painter needs excellent penetration into the substrate, he generally needs a lower PVC (Pigment Volume Concentration) first coat, such as a sealer or primer/sealer.” Higher PVC products yield stronger hiding qualities. Stain sealing is a job for lower PVC or higher binder content, and possibly specific additives to help block or tie up the stain. Knowing the substrate and choosing between solvent- and water-based primers are key in selecting the correct primer or sealer, advises Hardy.

In terms of opacity and hide, primers are generally not designed to work like topcoats. In terms of function, however, a special subset of primers called “sealers” can manage stains in a single coat, says Kinnen. “It depends on the substrate and the problem to be covered. It’s important to note that primers don’t have to be opaque to work well. If the shadow of a water stain shows through the sealer primer, that’s not really a concern. The sealer primer has sealed the stain from bleeding through to the topcoat. The topcoat is opaque enough to conceal the stain shadow.”

Hiding ability depends on the binder and the pigment in the formula. “The greater the amount of hiding pigment (titanium dioxide), the better the coverage,” says Champeny.

The Common Denominators
Primers and undercoats each play an important part in providing a quality finish coat. “It may be that the primer or sealer improves the surface and makes it paintable, improving adhesion of the topcoat,” says Hardy, noting that primers can also fill surface imperfections to improve topcoat appearance. Various primers are available to provide a range of benefits, including sealing the surface to improve topcoat gloss; enhancing impact resistance; and controlling corrosion.

Champeny points out that primers and undercoats are designed for good adhesion of subsequent coats, providing a system that has good adhesion and a uniform looking finish. Adds Martin, “Some are sandable, which provides a bond between coats, so surfaces won’t peel down the road. They may also allow you to see the surface and catch imperfections prior to applying a finish coat. The worst thing to do is go and buy $100 worth of paint, and then watch as stains start coming through.”

Improper application of primers and undercoats can trigger troubles. “The primer film needs to be applied to a clean, stable surface so that it can form the correct film,” advises Hardy. “If the air and surface temperature are too hot, the solvents or water will evaporate too quickly, preventing good film formation. If the temperature of the air and surface are too cold, the film may not form correctly.”

Incompatibility between primers and topcoats can result in lifting, wrinkling and lack of adhesion, says Hardy. “Over-thinning the product, applying too thin or too heavy and recoating too quickly — all of these can lead to a paint problem,” adds Champeny.

Primers and undercoats are applied to a variety of substrates. “Any surface being painted for the first time and surfaces that will be subjected to tough environmental conditions should be primed,” advises Hardy. “Surfaces that are glossy, hard and have a low surface energy should be primed, and it is important to seal or prime surfaces that are porous or that may bleed or rust.”
No one primer is perfect for every surface. “Some block rust like an iron oxide, others seal drywall like a vinyl latex, others are meant to lock in tannin stains,” explains Martin. “Your best bet is to read the label and know what you are applying.”

To thin or not to thin?
The issue of thinning primers draws proponents and opponents alike. All agree, however, on one point: Follow the manufacturer’s label recommendations where thinning is concerned. “Thinning a primer can and will change the application properties,” says Hardy. “Usually the primer manufacturer will state on the label if the primer should be thinned, with what and how much.”

Thinning or adding solvent reduces the viscosity and solids of the primer, which can aid penetration, improve flow/leveling and help atomization when spraying. “But it may also affect cling and build of the wet primer film during application,” says Hardy.

Sherwin-Williams doesn’t recommend thinning primers. “If you pick the right primer,” says Kinnen, “there should be no need to thin the product for improved flow or penetration. Thinning can compromise the integrity of the product, except when the primer is thinned — according to manufacturer’s instructions — for use with spray equipment.” Paints typically are designed to give optimum performance without thinning, adds Champeny. “Most solvent-thinned products are formulated at the VOC limits and should not be thinned under normal application conditions.”

While most primers generally have a high pigment content or PVC, sealers are an exception. “A sealer, as the name implies, renders the surface less porous,” says Champeny. “It makes the finish coat more uniform. Sealers have a high percentage of resin or binder and often little or no pigment.”

Made in the Shade
Adding tint to primers is a delicate operation. Hardy says XIM generally asks that no more than two ounces of tint be used per gallon of primer. “Adding too much tint can affect the dry time, cure time and the film formation of the primer. The painter should follow the primer manufacturer’s recommendations.”

Some deep-tone primers are designed to take as much as 8 ounces of tint, says Champeny. “The performance of some primers may be compromised by addition of tints. It’s best to follow manufacturer’s recommendations.”

But, according to Kinnen, there is now another option. It’s no secret that with deep-tone or vivid accent colors — which are gaining in popularity as consumers become increasingly more interested in dominant, bold colors — pigment opacity can pose problems. Kinnen says that these deep-tone or vivid shades don’t hide as well as their lighter-toned brethren. By virtue of their chemical composition, deeper pigments are actually more transparent and so do not completely hide the substrate. Vivid shades over white primer reflect more “white” or light; vivid shades over tinted primer may offer more “depth” of color, but could still be transparent, even with multiple coats.

Many a painting contractor knows the headache of explaining to a customer why the red wall doesn’t look just like the red color strip, and why it will take more time — and thus, more material-and-labor dollars — to make it so. “That’s why Sherwin-Williams has developed the ColorPrime Primer System,” says Kinnen. This patent-pending system features a variety of grey shades designed to be synergistically paired with topcoats to take the guesswork out of the primer-selection process. “This sort of predetermined pairing saves contractors time and money by reducing the number of topcoats, and helps ensure that customers are satisfied with the final results,” says Kinnen.

While use of primers never hurts, no surfaces “require” them, says Champeny. However, he concludes, “As a general rule, the best-performing, best-looking and longest-lasting paint systems include a primer as the first coat.”