Primers & Undercoats
In order to get the best painting results, it’s often necessary to use a primer or sealer before applying the paint. But deciding when to use a primer or sealer and determining the type of product to apply are mysteries to many of us.
To get a paint job it is necessary to have a working knowledge of primers and sealers and how they can improve the appearance, performance and longevity of a paint job.
Benefits of Primers
Primers are specially formulated coatings that perform several valuable functions:
First, they help the paint adhere better to the surface that’s being painted. They do this by making the surface more uniform and accepting for the top coat, so that when the paint is applied, it can get a better “grip” than it would on the bare surface.
Second, primers help give the finished paint job a more uniform appearance in terms of color and sheen, thereby making it more attractive. This is especially true when the surface being painted is porous or uneven in porosity.
Third, some primers help prevent stains from coming through the paint from the surface below and ruining its appearance. This is especially important with acrylic finish coats, which otherwise are vulnerable to stain bleed-through. With each primer you use, it is important to understand whether or not stain blocking is one of its capabilities and, if so, what types of stains will be blocked.
Sealers Enhance Uniformity
Closely allied with primers are sealers. While primers are generally pigmented, sealers often are not.
The role of a sealer can be to seal a porous surface like weathered concrete or stucco, so that a finish coat can develop a uniform sheen or gloss. Sealers also help protect the finish coat on masonry from efflorescence and alkalinity. And some sealers are used on masonry to help to seal out moisture, in which case the masonry is often left unpainted.
When to Use a Primer or Sealer
To achieve a quality paint job, it is recommended a primer or sealer be used in the following circumstances:
1 when new wood, new drywall or any other surface that has never been painted before;
2 when repainting a surface that is uneven or badly deteriorated; and
3 when repainting a surface that has been stripped or is worn down to the original surface material.
4 Just like paints and other coatings, primers and sealers perform best when the substrate is properly prepared. Regardless of the application, the surface to be primed should alwaysbe clean and free of all dust, dirt, grime, loose or flaking paint and other contaminants before the primer is applied.
The necessity of using a primer or sealer — and the type of product to use — varies from job to job. Here are some general guidelines for common applications:
New Unpainted Wood. If the wood isn’t severely staining, it is recommended that either a quality acrylic latex or an oil-based exterior wood primer. In the case of severely staining woods, use an oil-based stain-blocking primer. You should prime and paint bare wood within two weeks to keep wood fibers from deteriorating and reducing adhesion.
Weathered and Unpainted Wood. Use either a quality acrylic or oil-based primer. Firstly scrape and sand the wood thoroughly before priming because the deteriorated wood fibers must be removed, or adhesion will be compromised. Also, the primer should be applied shortly after surface preparation.
Previously Painted Wood. All loose paint should scraped off and rough edges feather-sanded. Any bare spots should to be sanded thoroughly and dusted off. In addition, as much chalk as possible should be removed before priming. If the old paint is very chalky, and all the chalk cannot be removed, use an oil-based primer. If the old paint is in sound condition and is still adhering well, priming can be beneficial, but is not necessary.
Stucco and Other Masonry. On new masonry, or older surfaces that are very porous, use an acrylic masonry sealer or primer. In a repaint situation, use a sealer only where the old paint has been removed during surface preparation or through weathering.
Aluminum or Galvanized Iron. Remove any white, powdery oxide, using a non-metallic scouring pad or steel wool (be sure all steel particles are washed off). Then apply a corrosion-inhibitive metal primer to all exposed bare metal.
Ferrous Metals. Remove any rust by wire brushing. Rinse, let dry and then apply a acrylic or oil-based rust-inhibitive primer. Two coats of primer will provide added protection against future rusting.
Most interior primers are designed for very specific applications. They often come in both acrylic and oil-based formulations. Keep in mind, however, that acrylic products are much lower in odor, which is a significant advantage on indoor projects.
Here are some common types of interior primers and the applications in which they should be used:
Primers - Sealer - Undercoats. While they are called primers, these coatings actually serve as pigmented sealers on drywall to give the top coat of paint a uniform appearance. They also help enhance appearance by providing additional hiding.
Stain-Blocking Primers. These interior primers protect the new coat of paint against bleed -through of such stain-producing agents as dirt, ink, crayon marks, smoke residue and waterborne material. Both acrylic and oil-based products are available. Oil-based products are most effective for blocking water stains.
Bonding Primers. These specialty primers are designed for use on very slick or glossy surfaces such as glass, ceramic tile and Formica™-type laminates. For maximum adhesion, sand the surface first with fine (#220) sandpaper.