Cedar and redwood contain water-soluble extractives that move through wood fibers the moment moisture contacts them. Apply a standard latex primer to bare cedar, and you will see it within hours: reddish-brown streaks emerging from the grain, from knots, from end cuts, and from sap pockets, discoloring the primer before you ever apply a topcoat. This bleed does not stop at the primer layer. It migrates into the finish coat, producing stains visible through the final color, especially under lighter paint colors where the amber and brown tones of tannins show clearly.
The problem is not a sign of bad primer, exactly. It is a sign of the wrong primer for the substrate. Standard acrylic primers are water-based, and tannins are water-soluble. Running water across wood loaded with tannins and expecting the tannins not to move is like trying to contain a stain with a wet sponge. The extractives follow the water vehicle right through the primer film and into the topcoat. Solving it requires a primer whose vehicle does not mobilize those extractives in the first place.
What Tannin Bleed Looks Like on Exterior Paint
Tannin bleed has a specific appearance that distinguishes it from other primer or paint failures. The stains are brownish to reddish-amber in color, following the grain pattern of the wood. They appear heaviest where the wood has the most tannin concentration: around knots, along sap runs, at end cuts where the wood fibers are open, and on the faces of younger heartwood where the extractive concentration is highest.
On fresh cedar or redwood siding, bleed typically appears within the first 24 hours after priming, especially if overnight humidity is high or morning dew forms on the surface. Moisture from the environment is often enough to activate the tannins even before the topcoat is applied. By the time you topcoat the surface and step back to evaluate the job, the staining is already locked in under the finish.
The intensity of bleed correlates with the tannin content of the specific board. Western red cedar is one of the highest-tannin species used in exterior siding, with heartwood carrying substantially more extractives than sapwood. Clear heart-grade cedar bleeds more aggressively than low-grade material because the heartwood concentration is higher. Redwood heartwood behaves similarly. Both species look beautiful on an exterior, but both demand a different primer approach than pine, fir, or engineered siding.
A simple test to evaluate your risk level: wet a small section of bare wood with water and watch what happens in the next few minutes. If the water pulls an amber or brownish color out of the wood, you are looking at active tannins, and a standard water-based primer will produce the same result across the entire surface.
Why Standard Primers Fail on Tannin-Rich Wood
Standard exterior acrylic primers, including many that are marketed as stain-blocking, fail on cedar and redwood because their water carrier mobilizes extractives before the primer film has time to cure. A typical water-based primer has a dry time of one to two hours, but during that window while the carrier is evaporating, it is actively pulling tannins through the wood surface and depositing them in the primer film. By the time the primer appears dry, the staining is already embedded in it.
Latex stain-blocking primers can slow this process on substrates with moderate stain challenges, like nicotine residue or water stains on drywall, because those stain sources are relatively contained. Cedar and redwood present a different category of challenge. The tannins are not a localized spot stain. They are distributed throughout the wood structure, including in the grain pattern running behind every square inch of surface. The stain source is the substrate itself.
The other factor that amplifies failure is that exterior applications introduce environmental moisture that interior applications do not face. On interior surfaces, a water-based primer dries in a controlled environment. On exterior cedar, morning dew, overnight humidity above 85 percent, or brief rain can keep the wood and primer film moist for extended periods, giving tannins more time and more carrier to migrate. This is why a primer that might perform acceptably on cedar in a dry climate can fail visibly in coastal, humid, or Pacific Northwest environments where moisture exposure is chronic.
Oil-based primers do not share this problem because their mineral spirit or alkyd carrier does not dissolve water-soluble extractives. The tannins remain in the wood while the primer cures, and the oil film creates a barrier that subsequent water-based topcoats cannot breach. This is why oil-based exterior primers have been the traditional specification for cedar and redwood siding for decades.
Shellac-Based vs Acrylic Stain-Blocking Primers for Exteriors
Two categories of primer reliably stop tannin bleed: shellac-based primers and oil-based stain-blocking primers. A third category, water-based stain-blocking acrylic primers, can provide partial protection on light tannin situations but generally falls short on high-grade cedar and redwood heartwood.
Zinsser BIN is the most widely referenced shellac-based primer and the strongest tannin blocker available by common consensus among painting professionals. Its shellac carrier is alcohol-based, which does not mobilize water-soluble extractives, and its film-forming speed is fast enough that it seals the surface before tannins have a practical opportunity to migrate. BIN applies well by brush, roller, or spray, and dries in approximately 45 minutes, which is one of the fastest primer recoat windows in the category.
The important caveat for exterior use: BIN is appropriate for spot priming, specifically for knots, sap pockets, sap streaks, and end cuts, but it is not designed as a full-surface exterior primer. BIN is not formulated for long-term outdoor UV exposure as a stand-alone product. Applying it as the only primer coat on large exterior surfaces invites adhesion and weathering issues over time. The correct application is to spot prime every high-risk area with BIN, then follow with a full-surface coat of an exterior-grade oil-based stain-blocking primer as the primary field primer.
Zinsser Cover Stain Oil-Based Primer is the full-surface counterpart. It blocks tannin bleed across the entire face of cedar and redwood siding, adheres to glossy or chalky surfaces without sanding, and dries to recoat in approximately one hour. It is available in a low-VOC formulation, which matters in states like California and the Northeast OTC jurisdictions where standard oil-based primers are restricted or banned for consumer use. Check local regulations before purchasing any oil-based product.
KILZ Original Oil-Based Primer is a recognized alternative. It performs similarly to Cover Stain on tannin blocking and is widely available. Users and professionals note that KILZ Original tends to be quite thick and may require thinning with mineral spirits for spray application. The high viscosity is not a defect. It reflects the solid content that gives the primer its blocking capability.
Water-based stain-blocking acrylic primers, including KILZ 2 and Zinsser Bulls Eye 1-2-3, are not the right choice as the primary primer on heartwood cedar or redwood. They may suppress light bleed on sapwood or lower-grade material, but they should not be trusted on high-tannin surfaces. If budget or regulatory constraints make oil-based primers unavailable, the BIN spot prime plus a quality acrylic stain-blocker as the full coat is a better approach than relying solely on a water-based primer.
How Many Primer Coats Are Needed to Stop Tannin Bleed
The standard specification for fresh cedar or redwood siding is two primer coats before the topcoat. This is not a conservative or overly cautious recommendation. It reflects the practical reality that one coat of even an excellent stain-blocking primer does not provide enough film thickness to completely contain the volume of extractives present in high-tannin wood, particularly on surfaces that will face long-term moisture cycling outdoors.
The two-coat approach works as follows: apply Zinsser BIN or an equivalent shellac primer to every knot, sap pocket, and sap streak on the boards. Allow BIN to dry, which takes roughly 45 minutes at 70 degrees Fahrenheit. Then apply a full coat of Zinsser Cover Stain or KILZ Original Oil-Based over the entire surface. Allow the oil-based coat to cure for the manufacturer’s recommended recoat window, typically one hour for Cover Stain. Apply the second full primer coat, then topcoat when the final primer has cured.
For boards that have already experienced tannin bleed through a first primer coat, do not attempt to topcoat over visible staining. The stain will telegraph through the finish coat, especially under light or medium colors. Sand or lightly abrade the affected primer surface, then re-prime with BIN over the stained areas, followed by a fresh coat of the oil-based full-surface primer. Topcoat after the second primer coat has cured.
New cedar and redwood siding installed in very high-moisture environments, such as coastal Pacific Northwest regions, benefits from back-priming. This means priming the back face and all four edges of each board before installation. Back-priming reduces the moisture the wood absorbs from behind, which in turn reduces the moisture pressure that drives tannins to the painted surface. The added step significantly extends the life of the paint system and reduces the frequency of bleed recurrence over the years.
Apply primers when the surface temperature is above 40 degrees Fahrenheit and below 90 degrees, with humidity below 70 percent. For BIN, the surface temperature must be at least 15 degrees Fahrenheit above the dew point. These conditions matter because applying primer when the wood is cold and the humidity is high means the carrier takes longer to evaporate, giving tannins more time to migrate before the film seals. In cool, humid climates, plan primer application for the warmest, driest window of the day.
One coat of the right primer is far better than two coats of the wrong one. But for cedar and redwood heartwood, two coats of the right primer is the professional standard that delivers a result that holds for years without re-staining.