I don't know what the answer is but I have two observations:-
First, varnish doesn't seem to seal knots. I have varnished several things and found that the sap comes through. Knotting compound is shellac, I believe.
Second, sap always seems to come through on woodwork that catches direct sun, i.e. gets hot. Someone may know better, but I imagine you'd not need knotting if the woodwork is permanently out of strong sunshine,
Andy.