A.Mel Hines responds: Assuming that there are no problems with the framing, the most likely cause for the hairline cracks you describe is torn bedding tape. If the finisher allows the corner of his taping knife to cut, or "score" the tape, a hairline crack will often develop.

Caulking or applying another coat of compound to the crack are temporary fixes at best. The corner should be retaped, then finished again. Check the corners of your finishing knife to make sure they’re not sharp.

Another possible cause is that too much compound is being applied in one pass. Many finishers will apply compound to one side of an inside corner (covering the tape), and allow this first pass to dry before covering the other side of the corner with compound. If the second pass is too heavy, it may shrink back when it dries, and leave a hairline crack. Sanding this heavy coat until it just covers the tape will usually eliminate the crack. You should apply only enough compound to inside corners so that after it is lightly sanded, it barely conceals the tape.

It’s also important to prefill any gaps in the drywall at corner intersections. If the gaps exceed 3/8 inch, fill them with Durabond (U.S. Gypsum Co., 800/552-9785), a low-shrinkage compound that is mixed on site. Durabond is available in grades with setting times from 45 to 90 minutes.

Two other points to keep in mind: If you add water to standard compound, don’t exceed the manufacturer’s recommendations — generally one pint of water to five gallons of mud. And don’t use fiberglass mesh tape at inside corners unless you bed it in Durabond.

Mel Hines owns Atlanta/Pro-Serve, a ceiling and wall repair service in Atlanta, Ga.