In article , Ray K wrote:
When replacing my fascia boards, I primed them with a non-shellac-based
primer. Now, the knot holes show just a couple of days after applying
two finish coats. Can I get away with spot priming the knots with
shellac right over the existing finish coats, then applying new finish
coats? Or must I first sand the knotted areas down to bare wood?

I'd sure try spot priming first, to see if it works, before going to the
effort of sanding down to bare wood.