What Is The Difference Between Joint Compound & Plaster Of Paris?

When you need to make repairs to drywall, such as patching holes, filling cracks or recovering unsightly seams, you have a choice of repair materials. Joint compound is the best option in most cases, but sometimes — such as when you're repairing a plaster wall — you need something that sets more quickly and creates a harder surface that won't sand away. Plaster of Paris, a material traditionally used by artists, is such a compound. It has similarities to two other alternatives: hot mud and patching compound, but it isn't the same.

Hardening plasters, such as plaster of Paris, have been around for centuries. Until the end of the 19th century, they were mostly lime-based. A common recipe for plaster was to mix lime, aggregate and fiber — often horse hair. The addition of water initiates a chemical reaction that sets the plaster into a cement-like material that won't re-soften.

Along with the introduction of drywall in the early 20th century, gypsum became the preferred material for plaster. Gypsum, or hydrated calcium sulfate, occurs naturally, and when made into a powder and mixed with water, it forms a semi-solid surface that resembles plaster, but isn't as durable. To make a gypsum plaster, manufacturers add cross-linking chemicals to bind the mixture. Without these hardening chemicals, the gypsum paste is called drywall joint compound, or mud — with them added, the mixture variously becomes patching compound, plaster of Paris or hot mud.

Tip: Hot mud is "hot" because the chemical reactions of the hardening chemicals give off heat.

In drywall installation or repair — or wall repair in general — you have a selection of patching options. All are available in powder form, and non-setting varieties also come pre-mixed.

• All-Purpose Joint Compound — You typically buy this product pre-mixed; it comes in 1- and 5-gallon tubs, and, for large jobs, plastic bags packed inside cardboard boxes. This is the go-to product for virtually every drywall installation and repair purpose. It takes anywhere from 6 to 12 hours to dry, depending on the humidity and temperature, and it doesn't set.
• Topping Compound — A lightweight version of all-purpose joint compound, topping compound is the product professional drywall installers use to top off taped joints. Because it's lightweight and quick-drying, it's used only for the final coat. Topping compound is also used for texturing.
• Hot Mud — Hot mud comes as a powder and begins to harden as soon as it's mixed with water. The setting time is particular to the product you're using; some set in 10 to 20 minutes, while others take an hour. Drywall pros use hot mud to shorten the wait time between coats and to make stronger joints.
• Plaster of Paris — Modern plaster of Paris formulations may or may not contain gypsum. Plaster of Paris is best for patching holes in plaster walls, because its characteristics are more likely to match the wall material. You seldom need plaster of Paris to patch drywall.
• Patching Compound — Made with calcium sulphate, limestone and crystalline quartz, patching compound sets in about 20 minutes and makes durable repairs on wood, plaster and concrete, and it can be used to level subfloors. It's a coarse material that is unsuitable for wall repair unless you cover it with something else, such as joint compound.
  

Tip: You can use mud or hot mud to patch holes in the drywall as well as fill holes and cracks in plywood and other types of wood. Because it sets, hot mud makes a more durable repair, but it has a significant disadvantage — it's difficult to sand. If you can, make cleaner patches in wood or wallboard with regular, non-setting joint compound.