Why am I still seeing the wood knots?

If you happen to paint over a wood knot without sealing it first, this is what happens...

To prevent this you'll want to apply a shellac based primer (the most popular one is called BIN). This primer will act as a sealant and will prevent the wood sap from bleeding through the paint. For best results I recommend applying at least 2 coats of this primer (although 3 coats is safer for really tough stains).

This primer is available in a spray can, a quart size paint can or regular size paint can and requires approximately 15 minutes of drying time between coats.