Joint compound, or mud, is a paste-like building material used to finish drywall. Generally, you should mud drywall three times, waiting at least 24 hours between each coat to allow the joint compound to dry fully if you're using ready-mixed joint compound. But there are times when you must mud drywall many more times to hide ridges or to fill deep depressions.
Function
In an optimal situation, three separate coats of drywall mud are needed. With the first coat, embed paper drywall tape over the seams. With the second coat, cover the paper tape with a thick application of joint compound. With the third coat, leave the surfaces polished and smooth by applying and immediately wiping off a thin layer of joint compound. During each step, also coat screw heads and fill any dents with joint compound.
Dents
Sometimes large dents require more joint compound than three coats can provide. In these situations, keep applying joint compound until the dents are full and flush with the surrounding surface. The problem with this method is that you must wait for the joint compound to dry fully between each coat. A better option is to use quick-setting joint compound. Mix it according to the manufacturer's specifications. Typically, this means adding water to the joint-compound powder until you achieve a toothpaste-like consistency. Apply the joint compound and let it set for the specified time. Setting times vary from five to 90 minutes, depending on brand and type, so choose a joint compound that will give you enough time to complete your project before hardening.
Ridges
As with dents, hiding a visible ridge might take multiple coats. Load up both sides of the ridge with a thick application of joint compound to camouflage the visible ridge. Repeat this process until you can no longer see the ridge. Use quick-setting joint compound to speed the process.
Considerations
The three-coat process is standard for professional drywall finishers, but beginners may have trouble creating smooth walls and ceilings in just three steps. Don't worry if it takes you more than three coats. Move slowly and carefully and clean the drywall knife frequently to avoid leaving lines and flaws in the layers of joint compound. If necessary, use 100-grit sandpaper to smooth the dried compound between coats and 150-grit sandpaper to perform a final sanding before paint.
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