· by Michael Blair, Nashville's Painters
Primer does jobs a finish coat is not made for. The Forest Products Laboratory’s Wood Handbook chapter on finishing wood lists three on bare wood: a primer seals the surface so extractives do not bleed through, gives the topcoats something to adhere to, and evens out the difference between the color of the wood and the color going over it. New drywall needs a primer for its own reason, covered below. A can marked paint and primer in one may be a perfectly good paint. Whether a separate primer goes on first depends on what it is going over, and on the surfaces below we still use one.
Which surfaces always need primer?
New drywall and fresh joint compound. The paper face and the compound over the joints take up paint differently, and the 1996 edition of the Gypsum Association’s GA-214, Recommended Levels of Gypsum Board Finish defines a drywall primer as one made to fill the pores and equalize that suction difference. It recommends priming the finished board before the final paint, and it has a name for what happens otherwise: joint photographing, the finished joints shadowing through the decoration. Light striking a wall at a slight angle greatly exaggerates surface irregularities, the document says, and where a wall faces strong sidelight or takes a gloss paint it recommends a skim coat over the whole surface as the most effective way to keep the joints from showing. For ordinary wall paint it describes the usual choice as a good quality white latex drywall primer with higher binder solids, applied undiluted. Every patch from our drywall repair work is primed before the color goes on. Old wallpaper that cannot come off is a priming problem of its own, and our post on painting over wallpaper covers when we leave the paper up and how we pick a primer its maker rates for it.
Bare wood, especially under a light color. The handbook places highly colored extractives in the heartwood of western redcedar and redwood and of hardwoods such as walnut and mahogany, and many of them dissolve in water. Rain outside, or a plumbing leak or high humidity indoors, carries them to the surface, where they dry as a yellow to reddish-brown stain. Its answer is a primer labeled to block extractives bleed, usually an oil-alkyd one, which it says blocks extractives better than latex primers do for now; a latex topcoat can go over it, and it asks for an oil-alkyd primer on end grain such as the cut ends of siding and trim. Knots are their own problem. Their oils and resins are chemically close to oil-alkyd paint, so according to the handbook an oil-alkyd stain-blocking primer cannot block them and latex formulations are ineffective too. It calls for shellac or a synthetic knot sealer on the knots alone, primed and painted over. When new cedar or knotty pine is part of an exterior painting job, we seal the knots before any primer goes on.
- New drywall and any patch or skim coat
- Bare wood, with the knots sealed first
- End grain on siding and trim
- Glossy or previously oil-painted surfaces
- Water stains and old leak marks
Glossy and oil-painted surfaces
Glossy oil enamel is the other surface we will not paint straight over with latex. Older trim may have been finished in oil, and that is not something to guess at from across the room, so we check a hidden spot before choosing the primer. When the result is unclear, we prepare the trim as if it were oil.
Sherwin-Williams describes its Extreme Bond primer as made for surfaces that need more adhesion than its standard primers give, hard, glossy ones such as tile, glass panels and plastic piping, with minimal surface preparation, and compatible with a full range of finish coats. On slick trim we use a bonding primer of that type, after cleaning the trim and scuffing the gloss. In a house built before 1978, that scuffing disturbs paint that may contain lead. EPA’s Steps to Lead Safe Renovation, Repair and Painting pamphlet is written for contractors and homeowners planning painting work in pre-1978 housing with lead-safe work practices, and our post on lead paint in older Nashville homes explains what that looks like on a job.
Stains, and why they come back
Wood shows the mechanism plainly. As the handbook explains it, water moving through the wood brings the extractives with it and leaves them on the surface when it evaporates, so a leak can stain painted wood trim or a wood ceiling indoors, and it names oil-alkyd stain-blocking primers among the paints that block that leaching. Where a latex stain blocker is used instead, the handbook says to let it dry at least 24 to 48 hours before the first topcoat, because extractives can bleed into the topcoat if the primer has not fully coalesced.
On a drywall ceiling we work the same way. Once the leak is fixed and the ceiling has dried out, the mark gets a stain-blocking primer before any finish coat, and smoke, nicotine and marker get the same treatment. We do not try to bury a stain under extra coats of ceiling paint. Bathroom ceilings bring a moisture problem of their own, which our post on paint peeling off a bathroom ceiling takes up.
When can you skip primer?
Repainting a sound, clean wall that already has latex on it, in a similar color, is where we usually leave out a full coat of primer. We wash the wall, fill and spot-prime the patches, and go straight to the finish coats. That is how an interior painting repaint of ours runs when the old paint is in good shape.
Outside, sound paint is much the same. The handbook says that when paint has weathered evenly enough to expose the primer, there is rarely much preparation beyond mild washing before refinishing, and it traces the chalky look of old paint to degraded resin and loose pigment, which we wash off before anything goes on. Two topcoats over the primer is its ideal system, and with only one it puts the expected service life at about half.
Bare spots are a different matter, and the look of the wood is no guide. Chemical changes begin within a few weeks outdoors, yet wood weathered for only 2 to 3 weeks appears unchanged. In the Forest Products Laboratory test on western redcedar, paint on boards left out 16 weeks before painting peeled within 3 years, boards left out 1 week peeled after 13 years, and boards painted without weathering showed no peeling after 20 years. The handbook wants low-density species finished as soon as possible after installation, or primed before it, while denser Douglas-fir and southern yellow pine lost little adhesion until 3 to 4 weeks, and new smooth-planed wood should get its first coat within 2 weeks of installation. We spot-prime bare wood on the visit we expose it. If your siding or trim has worn through in places, ask us for an estimate.
Sources
- USDA Forest Service, Forest Products Laboratory, Wood Handbook (FPL-GTR-190), Chapter 16: Finishing of Wood — What primers do on wood; extractives in redcedar, redwood, walnut and mahogany and the yellow to reddish-brown stain; oil-alkyd blocks extractives better than latex; latex topcoat over oil-alkyd primer; end grain; knots need shellac or a synthetic knot sealer; 24 to 48 hours for latex stain blockers; mild washing and chalking; two topcoats over primer and the halved service life with one; preweathering test, species timing and the 2-week first coat
- Gypsum Association, GA-214-96 Recommended Levels of Gypsum Board Finish (1996 edition) — Drywall primer equalizes suction between paper and joint compound; primer recommended before final finish; joint photographing; oblique light exaggerates irregularities; skim coat for severe light and gloss paints; white latex drywall primer with higher binder solids
- Sherwin-Williams, Extreme Bond Interior/Exterior Primer — A bonding primer for hard, glossy surfaces needing more adhesion than standard primers, minimal surface preparation, compatible with a full range of finish coats
- U.S. EPA, Steps to Lead Safe Renovation, Repair and Painting — Lead-safe work practices for renovation, repair and painting projects in pre-1978 housing
FAQ
When Primer Is Actually Necessary — questions
Is paint and primer in one good enough for new drywall?
We would not rely on it. The Gypsum Association recommends a drywall primer on finished board before the final paint, a product made to even out how the paper face and the joint compound take up paint. Without one, the joints can shadow through the finish, which shows most under light coming across the wall from a window. If a self-priming paint’s own label says it can be used as the primer on new drywall, follow that label; otherwise prime first.
How do I know if my trim is oil-based paint?
Have a painter test a hidden spot before anything is sanded. We do that on site before choosing a primer, and when the result is unclear we prepare the trim as if it were oil: cleaned, scuffed and given a bonding primer made for glossy surfaces. In a house built before 1978, the lead question comes first, because EPA’s lead-safe work practices apply to painting work that disturbs the old paint.
Why does the water stain on my ceiling keep showing through?
On wood, the Forest Products Laboratory explains that water carries colored extractives to the surface and leaves them there as it dries, and a stain-blocking primer is what holds them back. We treat a water mark on a drywall ceiling the same way: once the leak is fixed and the ceiling is dry, the mark gets a stain-blocking primer before the finish coat. If that primer is latex, the handbook gives it 24 to 48 hours to dry before the first topcoat.
Does wood need primer before painting?
Yes. Primer seals bare wood so extractives do not bleed through, gives the topcoats something to adhere to and evens out the color underneath. On redcedar, redwood and similar woods, use one labeled to block extractives bleed, usually oil-alkyd, and seal knots first with shellac or a synthetic knot sealer.