Last summer I screened in my front veranda, 8' x32'. The portion under the large screened 'windows" is about 18" high, made from tongue & groove pine from a box store. I applied 3 coats of white shellac to the knots before applying 3 coats of solid white oil based stain. Later in the summer, I noticed that some knots were showing through the stain, so I applied 2 more coats of shellac and 2 more coats of stain. I have now noticed that just about all of the knots are showing trough again, more on the inside of this "wall" than the outside. The porch faces east, and these wall sections don't really get very much direct sun. Is there something I missed during construction? More importantly, what can I do now to permanently cover the knots so they don't bleed through again?
Thanks for all replies.

What kind of shellac did you use? I know I use B-I-N for this which is a white pigmented shellac formula and never have any problems. Zinnser makes other seals like Bullseye 1-2-3

Thanks ballengerb1 for you quick reply. I used standard white shellac from my hardware store. If I buy the better kind you suggested, can I apply it now, over all of the other coats of shellac and stain that are on the wood?

Yes you can apply over the other coats but I'd scuff it with sandpaper first to insure a good bond.

Thanks ballengerb1 for your help. I shall do as you suggested. Much appreciated.
PS I tried to rate your answer, but the system said I must spread some reputation around before I can rate your answer again. I guess you have helped me before. Nonetheless, thanks for your fast and helpful advice.

You must rate 5 other folks before you can come back and rate me again. I do not recall that you rated me already so this could just be a glich.

Are the knots just showing through the paint or is sap oozing out of the knots?

Thanks hkstroud, the knots are simply showing up as brown circles that are obviously knots. There is no sap oozing out, just discolouration. If we had stained the project any other colour other than white, this would probably not be a problem.