Joint compound

A paste of gypsum powder and water that fills seams around sheets of drywall

plaster of parispipe joint compound

Joint compound (also known as drywall compound, drywall mud, joint cement or mastic) is a white powder of primarily gypsum dust mixed with water to form a paste the consistency of cake frosting, which is spread onto drywall and sanded when dry to create a seamless base for paint on walls and ceilings. When used for new walls, joint compound effectively eliminates blemishes from the surface of drywall, such as fasteners, damage, or drywall tape. Joint compound is used to finish gypsum panel joints filled with paper or fiber joint tape, corner bead, trim and fasteners, and to skim coat. It is also convenient for patching holes, bumps, tears, and other minor damage to existing walls. In North America, troweling joint mud on gypsum panels is a standard construction technique prior to painting wall and ceiling surfaces. Joint compound type and formula selection forms part of a drywall system that can be finished anywhere from a level 0 to a level 5, where 0 is not finished in any fashion, and 5 is the most pristine. A similar compound is used in sprayed on textural finishing for gypsum panel walls and ceilings that are pre sealed and coated with a joint compound. Until the last century, several different plasters such as veneer plaster and "plaster of Paris" have been used in 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.

Joint compound is comparable and contrastable with spackling paste. The similarities and differences are discussed at Spackling paste § Comparison with joint compound.

Ready mix joint compound is a pre made joint compound of drying type only 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.

Powdered joint compound is mixed with water immediately before use. Powdered drying or setting type compounds are available in similar combinations and consistency as the ready mix. This type of joint compound is commonly referred to as "hot mud" due to the exothermic reaction that takes place when the powder is mixed with water.

Drying type comes in ready mix or powdered form. Drying type joint compounds are vinyl based and harden by evaporation. Drying type joint compound is usually more forgiving than setting type joint compound and can be used for as long a period as needed while not dry or frozen. Drying type compound and all materials should be used at temperatures above .

Setting type comes only in powdered form and are mixed with water immediately before use. Setting type joint compounds are plaster of paris based and harden by chemical processes and evaporation rather than solely evaporation, giving it an advantage in filling holes and gaps that would take many days to dry out with shrinkage cracks if using the drying type compound. Another advantage to setting type compounds is that after curing, they do not soften when wet. Thus, they are better for moist environments, such as bathrooms. Setting type compounds are available in setting times ranging from 5 to 300 minutes and finish characteristics from tough and well bonded to soft and easy to sand. After setting any leftover cannot be resaturated and reused. All tools in contact with setting type joint compound must be very clean, or the compound may be set up prematurely. Setting type compounds can be used at temperatures down to .

Lightweight joint compound formulas weigh 25 30% less than conventional joint compounds due to ingredient changes and allows for easier sanding.

Reduced dust formulas cause the dust particles to clump together, falling out of the air sooner than regular formulas, thus reducing airborne dust.

Moisture and mold resistant formulas are available.

Tapeless drywall joint compounds can be applied directly to the joints without either paper or fibreglass mesh tape. Eliminating the taping steps can save about 30% of labour time for finishing drywall joints. Tapeless setting joint compounds are either reinforced by chemical glues or fibers. Tapeless drying type joint compounds can be mixed with a fiber additive which can be mixed into conventional drying type compounds to create tapeless joint compounds.

Firewalls, walls built to slow the spread of fire, with penetrations fill the gap around the penetration with fire safing material (fire wool) and the surface around the penetration is sealed with an additional " 1" thick fire rated joint compound that dries red in color. Drywall systems without penetrations can obtain ASTM C475 fire ratings using conventional joint compound at the seams.

Construction workers mixing or sanding drywall joint compound are often exposed to high concentrations of dusts containing talc, calcite, mica, gypsum, and in some cases, respirable silica; which have been associated with varying degrees of immediate eye, nose, throat, and respiratory tract irritation. Prolonged exposure over time to 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 in the drywall joint compound, workers may also face an increased risk of silicosis and lung cancer. Prior to 1977, joint compounds often contained asbestos fibers to enhance cohesiveness. Exposure to friable asbestos increases the risks of various severe health conditions, including cancer. In 1977 the Consumer Products Safety Commission banned wall patching (joint) compounds containing asbestos. For the reasons above and possibly others, constant use of a respirator is recommended by almost all drywall compound manufacturers and is required by some labor authorities.