Why Does Joint Compound Crack When Dried With a Hair Dryer?

Cracking

• Fast drying in summer heat and low humidity can affect drywall joint compound in the same way as a hair dryer. If the water in the mud's surface evaporates much faster than the mud deeper in the patch, the layers of material contract at different rates. Cracks form when the shrinking outer layer contracts over the stable inner layer, pulling the patch apart. Sanding and painting take place only after joint compounds thoroughly dry. In hot humid weather, some joint compounds used in taped joints might need 18 days of drying time before finishing, according to the U.S. Gypsum website.

Types

• All-purpose joint compound serves as the base for taping over joints between Sheetrock panels and as the foundation for a finish coat of paint. Using taping compound for covering joints, followed by a thin coat of fine-grained topping compound, improves both strength and appearance. All three types of joint compound use mixtures that harden through air-drying. The drying time the compound needs depends on air temperature and humidity. Slow drying helps prevent cracking. Joint compounds don't need hot temperatures, and set up properly at temperatures above 55 degrees Fahrenheit, according to the House Painting Info website.

Finishing Gaps

• Drying joint compounds can fill gaps of one-eighth-inch width or less if cured slowly, according to the U.S. Gypsum website. If used to fill large gaps, the mixture dries unevenly and cracks. A layer about one-sixteenth inch in thickness provides a good base for taping. Products called setting joint compounds that harden through chemical reactions can fill gaps and holes larger than one-eighth inch in width. Setting times vary from as little as five minutes to four hours. Setting joint compounds require no extra heat source and set up in temperatures as low as 45 degrees Fahrenheit.

Hot Mud

• Setting joint compounds containing plaster of Paris combine gypsum powder with ground gypsum that underwent heat-treating. Adding water to the powder rehydrates the gypsum and quickly forms a hard mineral matrix before enough moisture evaporates to cause cracking. Like concrete, this type of setting compound produces heat as a byproduct of the chemical reaction. Hot mud can generate temperatures high enough to cause serious burns, according to the U.S. Department of Health and Human Services Household Products Database. Water in your skin or eyes can react with the dry powder during mixing, causing irritation or severe chemical burns.