What is causing/caused these brown circular stains in my cabinetry?

Question

Recently moved into a 1950s-built home and have unearthed a number of brown, circular stains on the wood inside various closets. These rings don't appear on the plaster walls, but rather on the wood shelves or siding in the closet. Here is a picture of one such brown stain that appears on the wood along the top of the hallway closet.

I've seen these stains in a number of non-adjoining closets throughout the house. The brown ring is not wet or damp to the touch, nor does it smell funny. If I take a scraping tool and scrape off the paint the same pattern appears on the wood.

What's interesting is that the stain appears to permeate through the piece of wood. For instance, here's a picture of the top of a shelf in one closet - note the two brown rings:

And here's a picture taken from the underside of the same board - note the matching rings.

My first guess was that this was a water stain of some kind, but I am more doubtful seeing as it's always a ring (and not a full circle) and that it permeates cleanly from top to bottom of the wood. Is this a knot in the wood and has sap leaked out, perhaps? Is this mold? Something else? Any ideas?

Also, what do I do about these? Should I just paint over them? My concern is that it would be just dealing with the symptom of a problem; also, who's to say that the stain won't go through another layer of paint? Or do I need to rip out the shelves and side paneling and put new ones in? (The wood/interiors of the closets appear very cheap so it's not like I'm ripping out high quality, custom-built cabinetry.)

Answer 1

These are stains caused by resin from knots.

To cure this problem you will have to strip the paint from the knot and surrounding area and then paint the area of the knot with knotting solution this will seal the end grain of the knot and prevent subsequent staining.

Reprime the stripped areas of wood and then repaint the whole piece.

Answer 2

I'm pretty sure this is bleed-through of knots from underneath the paint. They have the shape of knots, and it looks like what you get when that happens.

If you want to be dead positive (or don't believe me) the only real solution is a scraper. Yes, its a bit on the nuclear side, but it will prove the existence of a knot underneath. Scrape off the paint in that area of wood. Pick an obscure spot to do so. Bring it down to bare wood, and you should see a knot underneath. Since the answer is to use a product like Kilz anyway to intercept the stain from bleeding through again, having to repaint that small area is no real issue.

Why can't you just paint over it? Well, you can, and for a time it will look like it worked, but the stain will come through regular paint. So you need something that a stain can't bleed through. In the old days, one might have used shellac as a stain killer, in fact, I think some stain killer products were based on shellac.