I am replacing some baseboards in my house. It is the most basic type: flat board 5-1/2" wide by 3/4" thick. I would prefer to buy primed board, but my local shops do not sell it for some reason (they have a great variety of more exotic moldings, but not what I need).
So I've been buying 10' x 5-1/2" x 3/4" pine boards (about 10$ apiece), installing, then painting them with two coats of latex paint.
As I discovered when the paint dried, some knots are visible through the paint (texture, not color), and even some ripsaw marks. I did notice these imperfections prior to painting, but they seemed minor and I was expecting that two coats of paint would smooth them out completely beyond noticeable.
They also sell "select" pine boards. They are generally more consistent in shape, fewer bends or twists, no chips, and no knots. But using this furniture-grade board for baseboards feels like a crime against the Nature and the common sense. So I am buying the non-select grade to keep it light on my consciousness, rather than to save a few bucks.
Should I sand the boards before installing them? What method of sanding is recommended? I do have a random-orbit sander. Or should I just give it another coat of paint? Is there a special primer for this purpose? I wonder what the pros do in this case. The old boards that I removed are not painted on the back, so the builders primed and painted them, and nothing shows through. I want to achieve the same result.