Painting previously varnished skirting - Knotting solution required?

[andrewpreece]

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.

[Mike Hall]

The title says it all really. The skirting is currently varnished pine and we want to paint it. I've given it a rub down enough to act as a key, but will I then need to treat the knots in the wood or will the varnish already have sealed the knots? Will I be OK painting over the (keyed) varnish, or must it really be rubbed down to bare wood. NB. The paint we've got is Dulux Realife Tough Satin, [in Barley White :-) ]

[Paul Mc Cann]

I would imagine that if the knots haven't made themselves known before they are unlikely to do so now