Pine usually requires an oil based primer to seal up any knots. Apply with a roller, brush, or spray it on like Spraygunn said. After the primer dries, sand the post to remove grain that has raised up. Then clean off dust and apply 2 coats of paint.
Actually shellac is one of the best sealers for pine knots.
You do not want an oil based primer unless you plan to apply an oil based finish. A better choice is an alkyd. Alkyd films can be painted over with either latex or oil based finishes. They come suspended in oil-based solvents so people confuse them as being oil-based primers. Not so.
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