We have unfinished pine furniture that we painted white. Over the years there are yellow stains coming through the paint. What happened? How can we repaint them and prevent the yellow stains from coming through again?

We have unfinished pine furniture that we painted white. Over the years there are yellow stains coming through the paint. What happened? How can we repaint them and prevent the yellow stains from coming through again?
I'm no expert but I think you'll have to sand them and put shellac on the knots. I had to do it with my fireplace mantle cause the knots were coming through. Wait till it dries and paint it over again. Hope I've been of some help.