Tannins are polyphenols, or secondary metabolites of plants, that play diverse roles in plant physiology, like defense against UV radiation, pathogens, microorganisms, and insects. Tannic acid is a specific form of tannin, and its concentration varies
not only between different plant species, but even within the same species based on individual growth environment. Generally speaking, however, lighter-colored woods like maple, birch, and aspen contain less tannic acid, while darker-colored woods
like oak, cherry, mahogany, and walnut contain more.
Higher levels of tannic acid can cause discoloration in the staining and/or coating processes. This happens because:
- Water, including in water-based finishes, can draw water-soluble tannic acid to the surface.
- Contact with water and iron (like nails) causes tannic acid to take on a gray or blue hue.
- As an acid, it greens or browns when exposed to alkaline materials, like ammonia, which is a common ingredient (pH regulator) in water-based finishes.
Even experienced, high-quality contractors can run into problems with tannic acid discoloration, also called tannin bleed or tannin pull. Although tannin pull doesn’t always occur and can occasionally be difficult to spot when it does, the result
is often sanding the floor twice (while still getting paid once), making an ounce of prevention with the strategies below definitely worth a pound of cure.
Photo credit: NWFA, “Problems, Causes and Cures 2018”
1. Take extra care preparing the floor.
Remove contaminants that may react with tannic acid by deep cleaning the floor before sanding according to accepted NWFA/MFMA procedures.
Most importantly for wood species that contain high levels of tannic acid, be sure to remove ALL dust, first by vacuuming, and then by tacking with a dry microfiber mop. If wood dust is not entirely eliminated, it can build up in the application
puddle of the finish to the extent that even a light applicator mark leaves a darker layer of finish.
2. Use a tannin-blocking sealer…correctly.
By and large, use of a tannin-blocking sealer is always recommended by manufacturers of water-based finishes to help combat alkalinity and moisture in their formulations. It is arguably most crucial when working on a color-sensitive project.
These days, when we hear complaints of discoloration, it usually involves light-colored stains, such as grays and whites. On floors stained dark brown, tannin pull may be present but not visible.
The first coat of finish should be applied over the tannin-blocking sealer before any buffing, screening, or fine sanding of the grain raise, as this will provide a surface for finish coats to build while preventing or minimizing discoloration. Dry
time—between application of stain or paint and sealer, as well as between application of sealer and finish—is also critical to avoiding tannin pull.
- For HyperTone™ Stains, it is strongly recommended that initial coats of sealer not be applied
for 24 hours, even if moisture readings are back to the original baseline.
- For Basic Coatings® sealers and finishes, each coat should dry a minimum of 2–3 hours (in ideal conditions), with the goal of reaching original baseline moisture readings before
proceeding to the next coat.
Need help selecting a tannin-blocking sealer? Basic Coatings® offers Raw™ Sealer,
which is recommended specifically when preservation of the natural look of white oak is desired. We also offer Lock ‘N Seal™ for
all other use cases working with species with heavy tannic acid.
3. Avoid puddling the finish, application trails, and applicator set marks.
Too much or not enough overlap in the working puddle can leave uneven areas on the floor or streaks, so even coats are critical. Achieve optimal results with a T-bar by following the directions in the video or steps a–c below.
- Edge out about 6 inches from baseboard or wall with a paint pad or polyester brush. Stay within 2–3 feet of the finish line to avoid lap marks, streaking or thicker film build. Be sure to feather the edge.
- Starting about 1 foot from the head wall, pour a 4-inch wide line of finish the length of the floor going with the grain. Stop about 3 feet from the opposite wall. As close to the wall as possible, hold the applicator at a slight angle and drag
it in a “squeegee” or “plowing” type motion to move the puddle from one side of the room to the other. Overlap 2 inches of the last pass and pull parallel passes until the entire floor is coated. Brush out all turns,
stops, and other applicator marks brushing in the direction of the grain of the wood. Feather out turns with a painters pad after the T-bar has made the turn. Add finish to the puddle-line when needed to maintain a wet edge across the room.
- When 2–3 feet from exiting wall, begin to taper the puddle-line down to a nickel-sized bead. Back out of the room by wetting small sections of the remaining floor, brushing away from you in the same direction as the grain of the wood.
Using a roller instead of a T-bar? Check out an instructional video by clicking here.
By following these 3 tips, you can prevent tannin pull, even on color-sensitive projects. If you have questions about tannic acid discoloration or other issues you are experiencing, please click here, fill out the online form, and your Basic Coatings Regional Manager will reach out to you!