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Joint compound


Joint compound (also known as drywall compound or Mastic) is a white powder of primarily gypsum dust mixed with water to form a mud the consistency of cake frosting, which is used with paper or fiber joint tape to seal joints between sheets of drywall to create a seamless base for paint on interior walls. 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 5 to 210 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.


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