Joint compound

Joint compound is comparable and contrastable with spackling paste.[3] The similarities and differences are discussed at Spackling paste § Comparison with joint compound.

Drying-type joint compounds are vinyl based and harden when they dry by evaporation. A drying-type joint compound is usually more forgiving than the setting type of joint compound and can be used for as long a period as needed while not dry, but it must be kept from freezing.

Three main joint compound formula types:

1. Premixed taping compound is used for bedding and taping coat.
2. Finishing or topping compound is used for filling and finishing coats.
3. All-purpose joint compounds can be used in bedding and taping coats, filling and finishing coats.

Setting-type joint compounds contain plaster of paris that sets through a chemical hardening process rather than solely evaporation, giving it an advantage in filling holes and gaps that would take many days to dry out and have 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.

This type comes in powder form and are mixed with water immediately before use. 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 45 °F (7 °C).[4] Some drywall professionals use setting type mud for the first coat and a drying type for the thinner finish coat.

Walls built to slow the spread of fire are called a firewall and are sometimes built using special fire-resistant drywall. A special joint compound for use with fire-rated drywall is needed.[5]

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.

Mudding is usually done in three layers, and it is essential 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

• finish coat or topping coat which is very smooth. A topping compound is soft, smooth and easy to sand.[6] Some finish coat sanding 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.

Workers applying joint compound to drywall.

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 sprayed-on textural finishing for gypsum panel walls and ceilings pre-sealed and coated with a joint compound. The flexibility and plastic qualities of joint compound make it a very versatile material both as a sealer or finishing coat for wall surfaces and in decorative applications ranging from machine sprayed texturing to hand-trowelled or even hand-crafted and sculptural finishes. In North America, applying 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 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.

While joint compounds are used for bedding tape and initial layers overtop, applying topping compound for finish layer(s) that level and sand more evenly is best. Both require thinning for practical application.

Applying and sanding the compound is messy work, and finished surfaces, such as floors and air handling ducts, must be covered.

Pockmarks are a defect caused by air bubbles which form after the 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 is applied over a dense, setting-type compound. The moisture exits through the finished surface, making bubbles dry as pockmarks. The bubbles can be reworked while the compound is drying to get a smooth surface. Although additives exist to reduce pockmarks, their use is discouraged by drywall manufacturers.[7] However, these products reduce bonding, so they should not be used on the bed coat.

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.[8]

Prior to 1977, joint compounds often contained asbestos fibers to enhance cohesiveness.[9][10] Exposure to friable asbestos increases the risks of various severe health conditions, including cancer. In 1977 the Consumer Products Safety Comission banned wall patching (joint) compounds containing asbestos.[11]

For the reasons above, constant use of a respirator is recommended by almost all drywall compound manufacturers and is required by some labor authorities.[8]

• Spackling paste – Any of a class of pastes that fill small holes in walls and ceilings
• Plasterwork – Construction or ornamentation done in plaster or a similar material
• Calcium silicate – Chemical compound naturally occurring as the mineral larnite