Can I Use a Lightweight Joint Compound on an Exterior?

Function

• Lightweight joint compound, often used for coating embedded tape, comes in two forms: ready-mixed and powder. You can use ready-mixed immediately, while powder forms require water. Powdered compounds have various drying times, from five to 90 minutes. Ready-mixed lightweight compound, on the other hand, can take 24 hours to dry. For applications in outdoor environments, drying times for either type depend on airflow, humidity and temperature.

Exception

• You can use lightweight joint compound on exterior surfaces that are shielded from inclement weather. For example, the walls of some balconies and porches won’t encounter much direct moisture if a large overhang protects against driving rain or snow. Some builders install drywall on the underside of overhangs, creating ceilings that don’t encounter moisture at all.

Finishing

• Even if a wall or ceiling won’t encounter much moisture, seal the drywall well to prevent moisture from penetrating behind it. Use all-purpose joint compound to embed paper tape over the seams where drywall boards meet and to fill the depressions created by screw heads during installation. Coat your work again after the compound dries, this time using lightweight joint compound. Once it dries, sand everything smooth with 150-grit sandpaper.

Sealing

• Apply a coat of primer and a coat of paint. Choose exterior paints designed to handle moisture, temperature changes and the effects of sunlight. Finish the sealing process by filling all gaps with silicone caulk designed for outdoor use.