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The term priming is normally used to describe the coating finishing step underneath an opaque pigmented topcoat. And it can be compared to the sealer step underneath the clear topcoat. The function of a primer is to properly seal and fill the pores of the wood, along with providing opacity — or hiding power — in preparation for a pigmented topcoat.
In simple terms, a primer is formulated using a clear coat base and adding the following: Appropriate solvents to penetrate the substrate and seal the wood, fillers or silicates in the primer to build and fill in the pores of the wood, as well as to provide sandability; and titanium dioxide to provide opacity to the primer.
Types of primers
There are several categories of primer coating technologies that are commonly used in the wood coatings industry.
When choosing a primer, you must consider the physical and chemical requirements of the topcoat to properly choose the correct primer. In most cases primers must be kept in the same chemistry category of the topcoat to ensure a compatible system. As we will later note, the only exception would be urethanes and polyesters, or vinyl primers, which may be used under some lacquers and conversion varnishes. Here I will describe each chemical coating category and the recommended use for that coating type.
Standard pigmented lacquers
Standard lacquer materials are generally considered low solids, or the portion of the coating that does not evaporate and remains on the part. These coatings are typically used in lower-end production work. They are commonly used on furniture and case goods where moisture and harsh environments are not a consideration. These materials are the least protective and should not be used where deep pore filling is desired. In addition, ultraviolet (UV) exposure from sunlight may affect the finish over a period of time.
Pre and postcatalyzed pigmented lacquers
Pre and postcatalyzed lacquers are higher solids coatings than standard lacquers, and they will fill the pores of the wood more effectively. They have better chemical and moisture resistance, but they generally do not withstand UV exposure better than a standard lacquer. This category of pigmented coatings is used on furniture, case goods and low-end kitchen cabinetry.
Vinyl primers
Vinyl primers are a category of coatings that are normally used as primers only and are not commonly sold as straight vinyl topcoat chemistry. Vinyl primers are very versatile coatings. They can be used under lacquers, as well as conversion varnishes. They provide excellent moisture resistance, adhesion and flexibility. Vinyl can be precatalyzed or noncatalyzed. Precatalyzed vinyl normally can be sanded more quickly and more easily than noncatalyzed vinyl. Vinyl primers should not be used for coating systems that require an ultra high build to fill the pores of the wood substrate.
Pigmented conversion primers
Conversion primers are formulated with medium- to high-solids content ranging from 40 percent to as high as 60 percent volume solids. Thus on an average, half of the coating material remains on the substrate, which provides great pore-filling properties on many substrates, including medium density fiberboard (MDF). Conversion coatings are quite moisture- and chemical-resistant, making them a good candidate for use on case goods, architectural millwork and high-end kitchen cabinetry.
Most pigmented conversion varnish systems exhibit much better UV resistance than lacquer systems. Conversion varnish should never be applied excessively. They should not exceed 5 dry mil for the entire coating system, including primer and topcoat, to prevent checking or other coating failure after the product is fully cured. For this reason it is not recommended that conversion varnish be used over substrates that require ultra high build of primer and topcoats which must exceed 5 dry mil.
Two-part pigmented urethanes
In some cases, two-part pigmented urethane primers and topcoats range a little higher in volume solids than conversion varnish. Some urethanes do not have maximum film build restrictions.
When using multiple coats of primer and topcoat, you can fill and sand the primer to a smooth level surface on most substrates. Two-part urethanes are very high performance coatings for use on heavy wear areas and harsh environments.
Some acrylic urethane formulations are an excellent choice on exterior products and countertops. Acrylics are also generally considered to be one of the most UV-resistant categories of coatings available in the wood finishing industry.
Pigmented polyester coatings
Many polyester coatings are formulated with up to 90 percent solids content; therefore, these coatings are ideal for high-build, high-gloss finishes, or for very deep pore filling.
Polyester is used to finish musical instruments, table tops or anywhere a deep, rock hard finish is required. Normally there are no maximum film build restrictions for these coatings. An application of two to three wet-on-wet coats may be applied up to 40 wet mil.
Polyester pigmented topcoats are normally susceptible to UV discoloration and may not be desirable for a pigmented topcoat. Two-part urethanes and polyesters are compatible in the same finishing system, thus allowing the finisher to choose a urethane to apply over a high-build polyester primer to achieve better UV resistance.
Water-based primers
Water-based pigmented primers and topcoats are formulated in several categories of finish chemistry. Many resemble the chemistry of a solvent-based lacquer or conversion varnish-like formulation. They are safer to use and are becoming more popular and versatile in recent years. These should be evaluated for use when safety and environmental compliance is a consideration.
Whitewood surface prep for paints and primers
It is often assumed that primer and painted products do not require the same detail in surface preparation as their stained and clear-coated counterparts. This assumption is totally incorrect.
Pigmented finishes often require a higher degree of surface preparation because every defect will show up in the final painted product if the surface is not prepared properly. To avoid problems with painted finishes, make sure that all parts are free from machining defects such as tear-out, chatter, chipping or misalignment of the parts. Any defects and voids should be filled, prior to priming, with polyester putty or other fillers that do not shrink. All filled areas should be sanded smooth and level, making sure they follow the contours of the product.
All substrate surfaces should be sanded as thoroughly as if you were staining and clear coating. As a general rule of thumb, it is considered a best practice standard to sand the wood substrate to the highest grit possible without over polish-sanding the wood, to ensure good mechanical adhesion.
Sanding to the finest grit possible will allow the finisher to apply lighter coats of primer to achieve good coverage. Less primer is often safer than applying thick heavy coats that may crack or chip more easily. The average grit for final whitewood sanding of primers ranges between 150 to 220 grit; however, often many factors, such as using water-based coatings or priming over MDF, may require whitewood sanding up to 320 grit aluminum-oxide abrasives.
Always be sure to check with your coating manufacturer and follow its recommendations for whitewood sanding procedures before starting the finishing process.
After the primer is thoroughly dry, and all the shrinkage has occurred, you will need to sand the primer smooth and level in preparation for the pigmented topcoats. Normally primers can be sanded with a grit range of between 220 to 320 grit. If pigmented topcoats are to be high gloss, you may need to sand the primer with abrasives up to as high as 500 grit.
Again, consult with your coating manufacturer for primer sanding recommendations. Always look at all primer sanded surfaces under inspection lighting to check for insufficient sanding and surface defects before topcoating.
Ed. note: Phil Stevenson is the founder of the American Wood Finishing Institute (AWFI) in Vineland, N.J.
author: By Phil Stevenson