Knots in siding

Well, when we repaired the siding last fall and found at the last minute that the cedar boards were just a shade too thick, we expected trouble from the pine siding we used instead. We had only time to waterproof one side and chose the rear.
A couple of dozen knots now show through the opaque stain. Standard cure is to cover the knots with shellac. Is there a better way than just to sand each knot, shellac it, and repair the stain when dry ?
-- Don Phillipson Carlsbad Springs (Ottawa, Canada)
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