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Q. My company is converting a garage with drywall on its walls and ceiling into living space.The joints have been taped with one coat of mud, but some of the tape is dry and loose, and the drywall's paper surface — which was never primed — has darkened and is fuzzy. Can I just apply new compound to the joints and continue the taping process? And is there a primer that should be used with old drywall?

A.Myron Ferguson, a drywall contractor in Broadalbin, N.Y., and the author of Drywall: Professional Techniques for Walls and Ceilings, responds: Sunlight — as well as the mere passing of time — will cause the paper face of drywall to yellow or turn darker, a process that begins in less than a week if the drywall is exposed to direct sunlight (see photo).

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The color typically bleeds through the compound when seams are taped, and it also bleeds through latex paint.

Seams and fasteners that were taped before the surface turned yellow usually stay whiter, so the newly painted surface becomes streaked.

If a wall or ceiling is not going to be finish-painted soon, it's a good idea to apply a coat of primer over the newly hung drywall, whether it is going to be taped or not. This will prevent the yellowing of the drywall's face paper.

In your situation, you should begin by removing any loose tape, then sand the surface to remove cobwebs and dirt, and maybe even wash the walls and ceiling with a damp sponge before repairing and finish-taping the drywall.

Next, prime with an oil-based or shellac-type stain-blocking primer; I like to use Kilz (Masterchem Industries, 866/774-6371, www.kilz.com). Since this oil-based primer raises the nap on the paper, you will need to lightly sand the surface when it's dry.

You can then use a latex finish paint over the prime coat, but be sure to give the primer plenty of time to dry first.

Also, when you apply the finish coat, make sure the humidity level is low and that there is plenty of air circulation so it will dry quickly; this will help prevent any more bleeding through.