Q: We had some closets built of wood several years ago, then painted them white. The wood had several knots in it, and these kept coming through the paint, even after it was painted several times. I was told to put clear shellac over the knots, which I did. But in spite of this the knot holes came through the next coat of paint. Can you tell me what I can do now?

A: Shellac is often used to seal holes, and it almost always works. In your case there may have been one of two problems: First, the shellac may have ben old (if it's more than six months old it should not be used). Second, the shellac may have been thinned too much, or it may have been applied in too thin a coat. And third, after three coats of paint it may not be able to stop the staining because of sideways migration of the sap which causes the stain. I would suggest sanding each visible knot stain with medium grit paper to get part of the old paint off. Next, apply a coat of pigmented flat white shellac-based stain killer over each spot (BIN and Enamelac are two widely sold brands). Then paint over this.

Q: Can you tell me what size louvers one should have in the attic for a proper flow of air circulation? Should these be left open in the winters as well as in the summer?

A: I assume you are talking about the vents required in the attic above the insulation when the insulation is on the floor of an unfinished attic. Experts recommend vents that are large enough to provide a total of at least one square foot of free opening for every 300 square feet of attic floor space. For best results there should be at least two vents - one at each end of the attic, and they should be left open all year round .

Questions about home repair should be addressed to Home Improvement Department, Do-It-Yourself Clinic, The New York Times, 229 W. 43d St., New York 10036. Only questions of general interest can be answered in this column.