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Backstory: We had a leak in our roof that damaged some of the ceiling drywall. When we put new drywall up, the pre-existing drywall "sagged" a bit lower than the fresh piece that was screwed into the ceiling joists. Because of this, I layered the joint compound so that the two would be as level as possible.

In the latest coat of joint compound, the compound is showing small cracks that I assume happen while it is drying (and shrinking?).

  • I've read elsewhere not to use sandpaper but some sort of sanding sponge. Confirm or Deny?
  • Do I need to use a different product to fill these gaps?
  • Would this be caused by too thick of a coat of joint compound?
  • What is the best way to build it up so that next time I can do this right?
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    "Sagging" concerns me, did you join the pieces of drywall at a joist? Did you use tape (paper or fiberglass mesh) to cover the joints? If not, you may have movement that's resulting in the cracking.
    – BMitch ♦
    Mar 3, 2013 at 14:55
  • When I described the "sagging", I'm talking about existing ceiling drywall (10-15 years old) that was installed by the builder.
    – rynmrtn
    Mar 3, 2013 at 17:25
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    If the old drywall had gotten so wet that it became misshapen or its thickness changed then you did not remove enough of the damaged area. In this case you really should go back to "Go" and start cutting damaged material back to to the first joist where the material is undamaged. (The easiest way to provide backer for the new drywall is to screw in new 1x2 or 1x3 wood to the side of the joist that you cut back to). In the case that you were replacing new against old that was truely a different thickness then you should have stopped and acquired the proper thickness or shimmed the backer.
    – Michael Karas ♦
    Mar 4, 2013 at 1:28

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Firstly, @Michael Karas answer addresses the "sagging"

Cracking: Premixed Joint Compound (JC) hardens by drying out. As such, it tends to shrink. I only use it for final top coats, where the shrinkage is mitigated and application layers are about 1/8 or so.

Setting JC is dry and mixed with water to a similar consistency as Premixed JC, but it hardens chemically (due to plaster of paris). It shrinks much less.

If your patch is dry and firm, you can continue to overcoat it with premix until you obtain a flat surface.

If it is not firm, remove offending parts until you have a firm base. Replace with setting JC, adding fiberglass mesh tape or other backing as needed. Bare JC shouldn't span more than 3/4 inch without a mechanical backing: Refer to Jon Raynors answer here

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