I meant to seal the knots on my trim with dewaxed shellac/sealcoat before priming and painting to prevent bleedthrough, but I got ahead of myself and primed already. Will going over the primer with shellac still work to prevent bleeding, or do I need to sand and start over? I know the shellac will stick, I just don't know if it will do any good.
Sure. The shellac makes a barrier that the tannins can't get through. If you shellac over primer, it's still effective. I have verified this through experience.
As a side note, an oil based primer (like Kilz Original) that blocks tannins by itself will, for most applications, be sufficient to block knots from bleading through. It's also easy to sand out brush marks with the oil-based stuff.