On Fri, 10 Apr 2015 09:45:33 -0700 (PDT), Abby
wrote:

On Friday, April 10, 2015 at 11:06:14 AM UTC-2:30, TomR wrote:
In ,
:
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.

TomR,
Because the poly is old, I assumed it was oil based but I could be wrong. Nevertheless, wouldn't you think 2 coats of Bin & 2 of white latex would do the job?
If it doesn't you have other problems. I would have sanded the door
and primed it with either Zinzer BIN or Original Kilz (oil
based)tinted to final colour if not painting white. My preference
would lean towards the Kilz, personally.