Knots are common in dimension lumber. But over time, they can interfere with the finish on a project The problem is there's more sap in a knot than in the surrounding wood. And if the knot hasn't been properly sealed, the sap can bleed through the painted finish and create a stain or dark spot.
Before applying a primer coat to a project with knots, the knots must firstbe sealed. This creates an impermeable barrier over the knot so that sap can't "bleed" through the primer and top coats.
The knot sealer I use is called Kilz, a pigmented oil-based sealer, see page 31.
There's no real trick to sealing a knot. Just coat the knot (both sides) thoroughly with the knot sealer. Then, when the sealer is dry, the entire board can be primed.
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