What Does Paint Primer Do?
Primers are formulated to adhere to the substrate and create a uniform surface for the finish coat. Primers stick to the surface; finish coats stick to the primer. This type of product penetrates unpainted surfaces, smoothing out any porosity. Primers for wood, whether they are latex or oil-base, contain specialized ingredients that topcoats don’t always have (or have in lesser amounts). Stain-inhibiting tannin blockers or preservatives that are found in primers, for instance, are designed to soak into raw wood and seal it so the tannins don’t bleed through and stain the finish paint. Primers for other surfaces have similarly specialized ingredients. On raw interior wood, oil-based primers ($35, The Home Depot) might be a better choice because of their surface penetration. On the exterior, a latex primer ($22, The Home Depot) will last longer because it’s more flexible and can expand and contract with changes in temperature instead of cracking. There are a number of different kinds of primers, including oil-base, shellac-base, and water-base, and each has its own specialty. Ask your paint retailer for a recommendation on which product to use on the surface you’re painting.
Do You Really Need to Prime Before Painting?
Many homeowners, in a rush to complete a project, skip the primer and go right to the color coats. This shortcut often backfires because it might take an extra coat of color to get complete coverage. Because primer is typically cheaper than paint, that represents both wasted time and money. Primer is specially formulated to bond to raw wood and to seal porous surfaces such as drywall. Drywall and drywall joint compound soak up paint fast, and if you’ve invested in high-quality paint, you want your investment on the wall, not in it. For best results, prime all drywall joint compound and let the primer dry. Then prime the entire surface of the wall. Primer typically dries rapidly so you can apply the finish coat sooner. A good primer saves time, effort, and money. Consider having your primer tinted to the color of your paint so the paint covers better. (Your paint retailer can typically do this for you.)
What Are Paint Sealers?
Closely related to primers, sealers are formulated to make a porous surface non-porous. They are also used to cover knots and mildew to keep them from showing through the finish coats. You’ll find oil-base sealers, as well as products with reactive resins and epoxies. Most of these products clean up with strong petroleum solvents. Use sealers when working with the following surfaces:
Any unpainted surface, including new plaster, drywall, and old woodwork that has been strippedBare open-grained woods such as oak and mapleBare woods, such as redwood, that bleed through or discolor paintLarge areas of wallboard joint compound or patching plasterMasonry surfaces like unglazed brick, cinder block, and concreteMetal surfaces (use an oil-base primer-sealer with rust inhibitors to prevent corrosion)
How to Choose the Best Primer Paint or Sealer
Primers and sealers come in an array of formulations. Choose the right one for your job using the following tips.
Best Primers and Sealers for Exterior Applications
New unpainted wood: If the wood species won’t leech tannins into the paint, use either a quality acrylic latex or an oil-base exterior wood primer. For woods that will stain the paint, apply an oil-base stain-blocking primer. Prime and paint bare wood within two weeks of installation to prevent the wood fibers from deteriorating and reducing adhesion.Weathered, unpainted wood: Sand the wood thoroughly before priming to remove deteriorated wood fibers, which will compromise primer adhesion. Remove the dust, then apply a quality latex or oil-base primer shortly after preparing the surface.Painted wood: Scrape all loose paint and feather-sand rough edges. Sand bare spots, remove all chalking with a damp cloth, and let the surface dry. If you can’t remove all the chalk, apply an oil-base primer. If the old paint is sound and adhering well, priming can be beneficial but is not necessary.Masonry: On new masonry, or older porous surfaces, use a latex masonry sealer or primer. On previously painted masonry, you need to seal only spots where the old paint has been removed during surface preparation or by weathering.Aluminum or galvanized iron: Remove any white, powdery oxide using a nonmetallic scouring pad or abrasive. Then apply a corrosion-inhibitive metal primer to all exposed bare metal.Ferrous metals: Wire-brush any rust, rinse, let dry, and apply two coats of a latex or oil-base rust-inhibitive primer.
Best Primers and Sealers for Interior Applications
Most interior primers are designed for specific applications and come in both latex and oil-base formulations. You might prefer latex products because the reduced odors are an advantage when working indoors.
Drywall primers: Latex primers are best for unfinished drywall. While these products are called primers, they actually serve as pigmented sealers over drywall to give the topcoat of paint a smooth surface to adhere to. This results in a uniform appearance. They also enhance the hiding ability of the finish coat by providing additional tinting in the undercoat. Stain-blocking primers: These primers stop bleed-through of such stain-producing agents as dirt, ink, crayon marks, smoke residue, grease, mildew, and water stains. Both latex and oil-base products are available, but oil-base products are more effective for blocking water stains. Shellac-base stain-blocking primers are heavily tinted to keep mildew from bleeding through the paint. Always wash mildewed areas thoroughly with a mild bleach solution, allowing the solution to remain on the surface for 20 minutes. Let the area dry before priming it. Vapor barrier primers: These primers are useful in bathrooms, kitchens, and other rooms that can harbor moisture. In rooms without plastic vapor barriers in the walls, these primers help minimize the passage of the moisture through the walls to the exterior, thereby reducing the likelihood the moisture will push the paint off the outside walls. Because they retain humidity, they also help keep indoor rooms more comfortable in winter. Latex enamel undercoaters: Using these primers under semigloss or gloss paint helps ensure that the topcoat of paint will develop its maximum gloss and have a uniform look. For best appearance, some manufacturers recommend a light sanding on the undercoat before applying the paint. Bonding primers: These primers are designed for slick surfaces such as glass, ceramic tile, and synthetic laminates. Sand the surface first with fine (220-grit) aluminum oxide sandpaper to improve adhesion. Concrete primer/sealer: Use this product to reduce the dust on interior concrete floors and to smooth the porous surface.
