Knots in wood have a high concentration of resin. This resinous substance will migrate into and discolor a paint film unless it is blocked by a sealer. Knot sealers act as an impervious barrier to prevent the migration of the resin.
Usually with age, knot bleeding becomes less of a problem, nevertheless, knots should always be spot-primed with a knot sealer, regardless. It will always be a challenge to block out knot bleed in very green wood. Some knots will not only discolor the paint, but will actually cause the paint to peel because the resin actually oozes out from the knot, pushing the paint off.
Knot Sealers: What doesn?t work
Latex stain blockers
Latex primers are great for total surface priming. They are flexible and breathable and will allow moisture vapor to pass through them. The relative porosity of the latex primer makes them unusable as a knot sealer.
Oil based stain blockers
Exterior oil-based primers are a good blend of flexibility and stain blocking, while remaining slightly breathable. They are my first choice for total exterior primer. Oil-based primers are better than acrylic latex primers for stain-blocking, but still not very effective for sealing wood knots.
Shellac as a knot sealer
Shellac comes in clear, amber (or ?orange?), or white pigmented versions. The standard knot-sealer that has been used over the years is amber or orange shellac. I presume that the addition of pigment (as in white shellac primer) may slightly reduce the knot-sealing capability of shellac. The problem with un-pigmented amber shellac is that, like most clear-coats, many paints (and even some primers) will not adhere very strongly to a clear-coat. This problem can be solved by transitioning from un-pigmented shellac to an intermediate ?tie coat? of pigmented shellac, which can be re-coated by any primer or paint.
Two coats are often needed to seal knot-bleed, and, in some isolated problematic knots, even two coats will not prevent bleeding over time. Because shellac is brittle and impervious to moisture vapor transmission, it is not suitable as a total primer for exterior use. With exterior knot-sealing, you must only spot prime the knot itself, while on interior wood, as a rule, you may prime the substrate in total, with a few exceptions.
The need for a total primer
The sealing qualities of shellac necessitates a subsequent total primer coat over the top in order to achieve paint uniformity when used as a spot primer (as with all exterior knot sealing). Failure to do so will result in a spotted, blotching looking painted surface. The spot-primed knots will look shinier and whiter than the unprimed areas. The way to avoid this potential problem is to prime the entire surface in total with a universal primer/sealer prior to finish, coating after spot priming and sealing the knots.
Knot sealing systems (spot priming)
Good: Two coats (spot prime) of white pigmented shellac
Best: Two coats (spot prime) of clear amber shellac followed by one coat (spot prime) of white pigmented shellac (this will add pigment to help transition to the next full primer/sealer coat)
Follow up after this spot-priming of the knots with a total coat of an interior/exterior universal primer/sealer. This primer coat can be tinted to approximate or match the finish paint color
Recommended Total Surface Universal Primer / Sealers
Zinsser Coverstain or 123 Bullseye
Xim 400 white or UMA primer
Zinsser BIN (interior only total surface priming)
Use the oil (Xim 400 and Coverstain) and shellac (BIN) primers with good ventilation during and after priming.
You must prepare the knot for primer by first removing any oozing sap by scraping, then wiping with mineral spirits, then sanding.
Some knots and some woods that are very resinous will bleed until they dry out with time.