In ,
Abby typed:
On Thursday, April 9, 2015 at 5:46:12 PM UTC-2:30, TomR wrote:

I want to paint a door trim that has a few coats of oil based clear
polyurethane. It is pine and therefore lots of knots.

If it already has 2 coats of oil based polyurethane, I can't imagine
that the knots would bleed through. And, I wouldn't think that you
would need any type of additional sealer or stain killer, and you
should be able to just paint the trim with any paint with no problem.

On a similar
project I applied 2 coats of Bin Shellac Primer Sealer and then
applied 2 coats of latex white paint.... however, the knots still
bled after a few months.

When you say "on a similar project" do you mean that the original
wood already had 2 coats of oil based polyurethane, and then you
applied 2 coats of BIN shellac primer sealer, and then 2 coats of
latex white paint (4 coats altogether), and the knots still bled
through? If so, I just can't imagine how that could be possible.

From what I've read, Bin it is the "best"
primer for knots. Should I apply an oil based primer before Bin?
Anyone with suggestions?

Yes, they still bled after a couple of months.

I can't imagine how they still bled through unless maybe the polyurethane
was water-based and not oil-based polyurethane. But, you said it was
oil-based polyurethane (2 coats), plus two coats of shellac (Zinsser BIN
shellac-based primer sealer), plus 2 coats of latex paint on top of that.

I believe you, of course, since you are there and I am not, but I just can't
imagine how the bleed-through could be possible under those circumstances.