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.
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