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Joint compound (also known as drywall compound or mud) is a white substance similar to plaster used with joint tape to seal joints between sheets of drywall, primarily in building construction. It is often referred to simply as mud or as joint cement
Drying type joint compounds are vinyl based and harden when they dry by evaporation.
Ready-mix lightweight joint compound is a pre-made form of joint compound designed for fast application and easy maintenance. The compound is a complex combination often including water, limestone, expanded perlite, ethylene-vinyl acetate polymer, attapulgite, and other ingredients. The delicate mixture of compounds gives it a creamy texture that spreads easily onto drywall surfaces and then hardens as the moisture evaporates. Drying type compound takes a long time to dry out and is used to fill holes or gaps and shrinks as it drys possibly producing cracks in thick applications. Ready-mix joint compound is usually more forgiving than the setting type of joint compound. It can be used for as long of a period of time as needed, and does not dry up unless left unattended for a long period of time, but must be kept from freezing. This type of compounds should be used at temperatures above 55 °F (13 °C) and all of the materials should be a similar temperature.
Powdered drying type compounds are available.
Setting type joint compounds come in powder form and are mixed with water immediately before use. This type contains plaster of paris and sets through a chemical hardening process rather than evaporation which gives it an advantage in filling holes and gaps which would take many days to dry out and have shrinkage cracks using the drying compound. Setting type compounds are available in different setting times from 20 to 90 minutes and types which bond extremely well and are very hard and types which are soft and easy to sand. Once mixed with water the setting type must be used before it sets, any left over is wasted and if not enough is mixed another batch is needed to finish making, and all tools must be very clean or the compound may set up prematurely. This makes the setting type harder and more time consuming to prepare but they set quickly. Setting type compounds can be used down to 45 °F (7 °C).  Some drywall professionals use setting type mud for the first coat and a drying type for the finish coat. Setting type compounds do not soften when they get wet thus are better for moist environments such as bathrooms. Professionals can tape in two coats instead of three.
Reduced dust formulas cause the dust particles to clump together falling out of the air sooner than regular formulas reducing airborn dust.
Moisture and mold resistant formulas are available.
Ready-mixed joint compound is most commonly used in hanging drywall for new or remodeled homes. Application is simple and easy, usually never taking more than three or four coats. When used for new walls, joint compound effectively eliminates all blemishes from the surface of the drywall, such as fasteners, damage, or drywall tape. Joint compound is used to finish gypsum panel joints, corner bead, trim and fasteners, as well as skim coating. In addition, it is also very handy for fixing minor blemishes or damages to walls. It easily patches up holes, bumps, tears, and other minor damages.
Often referred to as drywall taping mud, joint compound is the primary material used in the drywall industry by a tradesperson, or applicator, called a "drywall mechanic," "taper," or "drywall taper." A similar compound is used in various ways as a sprayed-on textural finishing for gypsum panel walls and ceilings that have been pre-sealed and coated with joint compound. The flexibility and plastic qualities of joint compound make it a very versatile material both as sealer or finishing coat for wall surfaces, and also in decorative applications that range from machine sprayed texturing to hand-trowelled or even hand-crafted and sculptural finishes. In North America the application of joint mud and drywall tape sealer and trowelled joint compound on gypsum panels is a standard construction technique for painted wall and ceiling surfaces. Until more recently in North America, and through the world, several different plasters such as veneer plaster and "plaster of Paris" have been used in a similar ways to joint compounds as fillers or for decorative purposes since ancient times, and the actual make up and working properties of these compounds is much similar. Modern ready-mixes or powder and water mixes are available in a wide range of styles from slow-drying to quick-drying to suit specific demands for use by contractors or decorators.
Mudding is usually done in three layers and it is important to use the correct type of mud for the first and last layer though a multi-purpose compound may be adequate for all coats: Bedding coat or taping coat where the mud is applied to seams and corners and paper joint tape is pressed into the mud (if using a fiberglass mesh tape the self-adhering tape is applied to the joints first and the mud pressed through the tape). The mud used here needs to adhere well and be strong and is called a taping compound; filler coat where the tape is covered and roughly smoothed; and the finish coat or topping coat which is very smooth. A topping compound is soft, smooth and easy to sand. Some sanding of the finish coat is usually required to get a smooth surface. Sheets of drywall usually have tapered edges to provide space for the thickness of the tape and mud at the seams.
While joint compounds are used for bedding tape and initial layers overtop, it is best to apply topping compound for finish layer(s) that level and sand more evenly. Both require thinning for practical application.
Applying and sanding compound is messy work and finished surfaces, such as floors, and air handling ducts need to be covered.
Pock marks are a defect caused by air bubbles which form after joint compound is applied. The bubbles are caused by the inability of moisture to be absorbed into the surface such as when the surface is already painted, has a layer of grease or cigarette smoke, or a drying-type compound applied over a dense, setting-type compound. The moisture exits through the finished surface making bubbles which dry as pock marks. The bubbles can be reworked while the compound is drying to get a smooth surface. Commercial additives are designed to reduce pock marks or the use of dish soap mixed into the mud. However, these products reduce bonding so they should not be used on the bed coat.
Construction workers who sand drywall joint compound are often exposed to high concentrations of dusts, talc, calcite, mica, gypsum, and in some cases, respirable silica. Some of these have been associated with varying degrees of eye, nose, throat, and respiratory tract irritation. Over time, breathing the dust from drywall joint compounds may cause persistent throat and airway irritation, coughing, phlegm production, and breathing difficulties similar to asthma. When silica is present, workers may also face an increased risk of silicosis and lung cancer.
Joint compound mixes manufactured prior to the 1980s often contained a complex mixture of several substances. Among the additives used were asbestos fibers, which provided cohesiveness. Exposure to friable asbestos increases risks of various serious health conditions, including cancer. Joint compounds manufactured from 1980 onward were required to have asbestos removed in favor of other compounds due to legislation to ban asbestos' widespread use.