Joint compound, or drywall mud, is a building material similar to plaster. Finishers use it to seal joints and hide screw heads after a drywall installation. Many types of drywall mud exist, but most finishers rely on all-purpose, ready-mixed joint compound, which comes in 5-gallon buckets. The slow drying time allows finishers to complete a project before the compound becomes unworkable.
Ready-Mixed Joint Compound
Allow ready-mixed joint compound to dry for about 24 hours after you apply it. When it is wet, it is gray; when it is dry, it is white. Heavy applications take a long time to dry, so only apply as much joint compound as necessary. If the weather outside is dry, open doors and windows to bring in fresh air. Position fans at one end of the room so air will move quickly in and out. If the outside air is humid, close the doors and windows and set up dehumidifiers. Generally, the faster the air moves, the quicker the evaporation process will be.
As ready-mixed joint compound dries, it becomes unworkable. Typically, you have 10 to 15 minutes to smooth joint compound after applying it. After that, the compound tends to streak when wiped, leading to a rough texture that will require heavy sanding later. In low-moisture or high-heat environments, you may have just a few minutes before the compound is difficult to work with.
Apply no more joint compound than you can handle within a few minutes. Don't apply joint compound over large areas or you risk compromising its smoothness. For example, if you are covering a long joint, finish one half at a time rather than trying to coat the entire joint at once.
Setting-Type Joint Compound
Setting-type joint compound has a different chemical composition than ready-mixed joint compound. Rather than drying through evaporation, setting-type joint compound dries through a chemical reaction, as concrete does, so the drying time is more predictable. Setting-type compounds are available with a variety of specified drying times, from five to 90 minutes, making it convenient for projects with tight deadlines. Follow the manufacturer's directions for mixing to ensure the setting-type joint compound dries within the specified time frame. The color of setting-type joint compound may not change as it dries, so poke a few areas gently to determine if the surface has hardened.