I just dealt with this problem. Use a shellac based primer to cover the stained area throughly. Don’t buy a spray paint can, get an actual can of paint and apply it with a brush or roller. It is extremely thin. So don’t put too much in your brush or it will splatter everywhere. I used the Zinnser BIN primer, it completely encapsulates the stain. Then follow up with a coat of Kilz stain blocking primer on top of that before painting over with your actual paint color. My co-worker calls this the belt and suspenders approach. If one fails the other will hold up. I can’t see the stains at all.