The Four Main Causes
Joint compound shrinks as the water evaporates out of it. That's just chemistry. The amount of shrinkage depends on how much water is in the mix to start with and how quickly that water leaves. Cracking happens when the surface dries and shrinks faster than the interior of the coat, so the dry surface tears itself apart trying to keep up.
Almost every fresh mud crack I've seen falls into one of these categories.
Cause 1: Coat Too Thick
The most common cause, by a lot. All-purpose joint compound is designed for coats up to about 1/8 inch thick. If you trowel on 1/4 inch or more in a single pass, the top surface skins over while the bottom is still saturated. The skin tries to shrink, the wet underside resists, and you get cracks.
The fix is to apply multiple thinner coats. Three thin coats almost always look better than two thick ones, and the total dry time is usually about the same.
Cause 2: Recoating Before Dry
If you mud, wait two hours, and recoat, you're recoating over a surface that may be dry to the touch but still wet underneath. The fresh top coat dries faster than the partially dried base, and the base shrinks under it, cracking the top.
Lightweight all-purpose compound typically needs 24 hours between coats. The bag will tell you. Skip ahead at your own risk.
Cause 3: Room Too Hot or Too Dry
Joint compound dries best between 55 and 70 degrees Fahrenheit at moderate humidity. If you're working in a 90 degree garage with the door open in summer, water is evaporating off the surface much faster than it can move through the coat. Same crack pattern.
USG, the company that makes Sheetrock brand compound, publishes application guidelines that recommend keeping room temperature stable for at least 24 hours before, during, and after application. (Source: usg.com) That guidance exists for a reason.
Cause 4: Mud Thinned Too Much
Some people thin premixed mud with water to make it spread easier, especially for skim coating. A little water is fine. Adding 20% or more by volume changes the mud's behavior and dramatically increases shrinkage. More water in equals more water that has to come out, which equals more shrink.
If you need a thinner mud for skim coats, buy topping compound instead of thinning all-purpose. It's formulated for thinner application.
How to Tell Which Cause You Have
Look at the crack pattern. Thick-coat cracks tend to be wide and follow the contours of where you laid mud thickest. They concentrate in valleys and butt joints where you piled material to hide imperfections.
Recoat-too-soon cracks show up everywhere uniformly because the whole base coat is drying out under the fresh top coat. They're often hairline and form a grid pattern.
Hot-room cracks look similar to recoat cracks but usually show up within hours of application, not the next day. If you watched the mud crack while you were still in the room, the temperature was the problem.
Thinned-mud cracks are often deeper and more dramatic than the others. The mud can pull apart down to the substrate.
Fixing Mud Cracks
Hairline shrinkage cracks under about 1/32 inch wide can be skimmed over with the next coat. They'll disappear. Don't bother scraping them out.
Cracks wider than 1/32 inch but less than 1/16 inch should be scored open with a putty knife corner, then filled with fresh mud and feathered. The score opens the crack enough that the new mud can lock in.
Cracks deeper than 1/16 inch or that go all the way to the tape or substrate need to be scraped out completely and recoated. Skimming over a deep crack just creates a thinner version of the same problem and it will come back.
For any crack repair on a wall that's going to be painted in a critical lighting condition (raking light from a window, for example), do an extra skim coat. Cracks have a way of telegraphing through paint even when they look fully filled.
Preventing Cracks on Your Next Coat
Three rules cover most situations. First, never apply mud thicker than 1/8 inch in a single pass. If you need more buildup than that, do multiple coats. Second, respect the dry time on the bag, especially in cool or humid weather where it can take longer than the label says. Third, control your environment. Run the heat or AC to keep the room between 60 and 75 degrees. Avoid drafts that dry the surface unevenly.
If you do all three and still get cracks, check the mud itself. Compound that's been frozen or stored too long can develop unusual drying behavior. Throw it out and buy fresh.