Drywall mud, also called joint compound, is a pastelike material used to cover the seams between boards of drywall in a process called drywall finishing. Drywall finishers use wide-bladed knives and paper drywall tape to cover cracks and to hide the screws that are used to attach the drywall to wood frames.
Ready-mixed joint compound, the most common form of drywall mud, contains water. After the finishers apply it to the drywall, the process of evaporation begins. The joint compound turns from gray to bone-white as it dries. When the water evaporates, the joint compound shrinks because the remaining particles take up less space. This causes slight, shallow cracks to develop across the surface of the joint compound application.
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Powdered joint compound doesn't dry through evaporation. Rather, it dries through a chemical reaction in a manner similar to concrete. This means that using powdered joint compound makes cracks less likely. Typically, however, the short drying time of powdered mixes makes it inconvenient for large projects because the mix hardens before you can finish the job.
Thin coats of joint compound rarely crack, but thick applications often have many cracks. These cracks are not a serious problem, though filling them will require the application of additional, thinner coats of joint compound. This is why drywall finishers typically perform three consecutive coats, each one thinner than the last.
The cracks that result from the natural drying process are typically just fractions of an inch and jagged in shape. A long, straight crack, however, is a different kind of problem. To fix long, straight cracks, apply a thick coat of joint compound over the entire crack. Apply a piece of paper drywall tape and wipe it smooth with a drywall knife. Once the joint compound dries, which typically takes 24 hours, apply another thick coat to hide the tape. This repair method should prevent the crack from reappearing.