Primers are specially formulated paint products that are used to prepare surfaces for the finish coat of paint. Their most important job is to adhere to the substrate while creating a uniform surface that is ready to receive paint.
Primers also act to seal pores in wood and other permeable materials as well as to prevent stains, knots, and wood tannins from bleeding through. When properly applied, primers can make your paint job last longer and look better.
When to Prime
Every unfinished surface—including wood, drywall, metal, and concrete—should be primed before painting. While it can be tempting to skip this step, the results are almost always disappointing. Paint applied to unprimed surfaces tends to peel, crack, and chalk more than paint applied to properly primed surfaces.
Previously painted surfaces may not require priming unless you’re switching between oil-based or latex paint, or the existing paint is failing. Always scrape and sand any deteriorating surfaces before applying primer. Remember, your paint job is no better than the preparation that goes into it.
While it used to be necessary to apply oil-based primers over oil-based paint and latex primers over latex paints, many primers today allow you to switch between them as long as you prepare the surface properly. When painting over interior oil-based woodwork with latex, be sure to sand or degloss the surface first, then paint with a bonding primer before topcoating with latex.
Painting over multiple layers of oil-based paint on the outside of older homes with latex paint can cause adhesion problems, so in that situation, you’re better to continue using oil-based house paint.
At first glance applying primer may seem like an unnecessary expense, but it actually saves money as well as time. A good coat of primer improves paint’s hide, or ability to cover, reducing the number of coats that are necessary to achieve a smooth finish. Primers can be tinted to match the paint color. Tinting improves the primer’s hide and smoothes the transition between primer and topcoat.
Types of Primers
Primers are available in oil, shellac, or latex-based formulas. Each type has differing properties and uses a different solvent for thinning and cleanup. Choosing which type to use is largely a matter of matching the primer’s characteristics to the project at hand.
These slow drying primers release volatile organic compounds in the air and require mineral spirits for cleanup and thinning. They produce a very smooth finish that does the best job of filling pores in bare wood while not raising the grain. Oil primers also provide a good barrier to keep tannins from certain woods from bleeding through.
Oil primers are good to use for:
- Unfinished wood.
- Previously varnished wood.
- Redwood, cedar, or other woods that tend to bleed tannins.
- Heavily weathered wood.
- Over existing paint that is failing due to chalking or cracking.
These fast drying, water-soluble primers have come along way in recent years and are now available in low and no-VOC formulas. Latex primers are not as brittle as their oil or shellac-based cousins and provide a more flexible finish that is resistant to cracking. This makes them suitable for priming bare softwoods, though test them first to see if they raise the grain or allow resin to bleed through.
Latex primers are the best choice for unfinished drywall, since they act to even out the texture and sheen between the wallboard and joint compound. They also allow water vapor to pass through, which can make them less likely to peel.
Use latex primers on:
- Unfinished drywall.
- Bare softwoods like pine.
- Masonry such as brick or concrete block.
- Galvanized metal, after proper cleaning.
Pigmented Shellac Primers
Shellac-based primers are fast drying and use denatured alcohol for thinning and cleanup. Though smelly and difficult to use, they are excellent at blocking stains and preventing bleed through.
Shellac primers should be used to prime or spot prime:
- Water, smoke, and rust stains.
- Knots, pitch pockets, and stubborn tannin bleeding.
- Wood, metal, plaster, or plastic when you need a fast drying product.
Application and Clean Up
All primers should be applied to clean, dry, grease-free surfaces. It is a good idea to lightly sand surfaces—followed by wiping off any dust with a tack rag or damp cloth—before applying primer. If the primer leaves the surface rough, lightly sand and dust it again before applying the finish coat. For best results, paint surfaces within a week after priming.
Make sure you have adequate ventilation when using any paint product. This means that there should be no more of the fumes in the room than if you were working outside. If opening the windows and using fans isn’t enough, wear a mask or respirator. You might also want to consider using one of the new low or non-VOC latex formulations when painting indoors.
- How to Choose the Right Paint Primer (video)
- Priming Before You Paint (article)
- How to Tint Paint Primer (video)
- How to Prime Exterior Wood Siding (article)
- Importance of Back Priming When Installing Wood Siding (video)