Masonite composite hardboard has a natural moisture resistance. When you install Masonite, the area of the hardboard penetrated by a fastener can absorb moisture. Moisture causes the area around the penetration to expand. This expansion causes the fibers around the penetration to loosen, weakening the Masonite. To avoid structural failure of a piece of installed Masonite, you need to waterproof the surface of the Masonite after installation. Lacquer-based waterproofing systems bond to the surface of the Masonite and provide adequate protection against moisture intrusion.
Prepare the surface of the Masonite. Remove high spots from the surface of the Masonite with 100-grit sandpaper. Wipe the sanding dust from the Masonite with a clean rag.
Pour waterproofing sealer into a hand-pressurized spray bottle.
Secure the lid on the spray bottle. Pump the handle of the spray bottle to pressurize the spray tank.
Slide a foam roller pad onto a roller handle.
Apply the waterproofing sealer. Hold the tip of the spray bottle wand 6 inches from the surface of the Masonite. Move the spray bottle wand from the top to the bottom to coat the surface of the Masonite. Roll the foam roller pad across the sprayed surface to provide an even layer of sealer to the surface of the hardboard. Run the roller pad along each exposed edge of the Masonite to complete the waterproofing process. Pour any remaining waterproofing sealer back into its container.
Allow the waterproofing sealer to cure for five to six hours before applying additional coats of sealer or paint.
Clean the foam roller and the inside of the hand-pressurized spray bottle with lacquer thinner after you waterproof the Masonite.