Thick mixtures of drywall joint compound cover the joint tape that binds the seams between sheets of drywall.
Drywall professionals slather and smooth drywall joint compound across drywall joints to seal seams and connect adjacent sheets of drywall. Additionally, drywall finishers use joint compound to create surface textures, such as popcorn ceilings and knockdown wall textures. However, joint compound isn't a one-size-fits-all product; drywall tapers and finishers choose from among several varieties of joint compound according to the compound's workability and drying time. If you learn about the different types of drywall compound, you can choose the kind that suits your project and skill level.
Dry Vs. Pre-Mixed
Drywall joint compound, also called "mud," falls into two general categories: dry mud or premixed mud. Aside from brand-specific, propriety additives, the only ingredient that varies between the two types of compound is water. Dry mud is available in sacks, like concrete. Premixed, or "ready-mix" mud, is available in 5-gallon tubs or plastic-lined boxes. Dry mud weighs less than premixed mud, is easier to transport and has a nearly endless shelf-life. Premixed mud is heavier, more unwieldy and likely to harden if left exposed to air. However, using premixed mud saves time and creates less mess than the powdery dry mix.
All-Purpose Compound
As suggested by its name, drywall installers use all-purpose compound for all phases of taping, particularly bed coats and first coats. All-purpose compound is generally the thickest, heaviest type of compound available. Although drywall installers can dilute all-purpose compound for use during final coats and skim coats, the heavy composition of all-purpose requires more sanding than lightweight compound mixtures. All-purpose compound is available either dry or premixed.
Lightweight and Topping Compound
The recipe for lightweight compound and topping compound results in a smooth, thin paste that requires less sanding than all-purpose compound. Lightweight compound and topping compound are easy to manipulate with taping knives. Drywall tapers use these compounds to apply thin, precise finish coats and skim coats of joint compound. Lightweight compound and topping compound are available as either dry or premixed products.
Hot Mud
Fast-setting joint compound, called "hot mud," contains compounds that accelerate the compound's setting time. Hot mud is available only as a dry, powder-form product. To avoid premature curing, drywall installers typically mix only small portions of hot mud at a time. Joint compound manufacturers produce hot mud according to setup time. The term setup time refers to the amount of time from mixing to full hardening. Hot mud setup times range from an impossibly hasty five minutes to a reasonably quick 45 or 60 minutes. Because standard mud takes several hours to dry at room temperature, hot mud offers the drywall finisher a significant time advantage.
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