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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.

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If you are going to paint the baseboards, might as well used 5-1/2" rippings of 3/4" particle board. For the knot bleed-throughs, there are special primers designed to seal off the resins. I'd sand, prime, and put a coat or two on while the baseboard is uncut and on saw horses. –  mike Aug 15 '13 at 15:29
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If you select boards with knots, there is a good chance that they will bleed through, even if you use a sealer or shellac based primer. The resins in knots is persistent.

Clear boards are usually used for moldings, including baseboards. You could also use MDF (medium density fiberboard) or solid vinyl composite molding. When painted, they are hard to distinguish from wood in most applications. (I think they look a little too smooth and I prefer wood). For painted applications, there is also fingerjoint molding, which is made of interlocked short pieces and is cheaper than clear boards. Much pre-primed molding is fingerjoint. Occasionally these joints may show if the quality is not high.

For wood or MDF, a light sanding with 120 grit paper is recommended to remove milling glaze and to give the wood a tooth to better take paint. An orbital sander is fine

Then prime and paint. Many prefer oil based primer on raw wood and MDF. Some vinyl says it can be painted directly, but I would also give it a sanding (or avoid it unless necessary, such as in outdoor uses).

Latex paint is fine for a finish coat, but it tends to look less smooth than oil (alkyd) based finishes. Adding a leveling agent like Floetrol helps, but nothing lays as smooth as a good oil based enamel.

(P.S. To me, using a really smooth straight clear board, in furniture or in molding, is appreciating and honoring the wood, the tree, nature. But to each his/her own.)

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Thanks for your insight. I am not against using MDF or composite molding, but I could not find it in the right size. I will be sanding and priming according to your recommendation. Regarding your P.S. though, I have to disagree. Fine wood is hardly appreciated if covered by coats of primer and paint, becoming indistinguishable from MDF or worse, synthetic composite. Fine wood needs to be clear coated and used in applications where it matters. Using it for baseboards is wasteful. –  user443854 Aug 15 '13 at 17:03
    
@user443854 - agreed –  mike Aug 15 '13 at 17:04
    
There is an interesting passage in the book House by Tracy Kidder. The narrator, a master carpenter overseeing the building of a house goes back and replaces a prominent trim board that has a knot in it because he knows that sooner or later it will show through the paint. He had a reverence for wood, even painted, and his personal code led him to waste an already installed board (and the labor) to honor the level of craftsmanship and finish he valued. –  bib Aug 15 '13 at 17:12
    
I haven't read the book, though amazon-look-inside reveals there was protracted disagreement spanning a dozen pages about knots in trim boards. From where I sit though, a deep reverence for wood has nothing to do with knots. –  mike Aug 15 '13 at 18:04
    
@mike There are disagreements (between home owners, tradesmen, contractors, architects) and it's worth a read. –  bib Aug 15 '13 at 18:18
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