Recognized for its density, strength and traditional wood appearance, Masonite hardboard is a partially synthetic alternative to solid wood. Masonite siding is primarily composed of wood fibers, glue resins and wax. Unlike natural wood, Masonite hardboard does not shrink, blister or swell. Masonite siding is, however, affected by moisture and is susceptible to deterioration. Holes are a prominent problem with Masonite siding. Promptly repair holes in Masonite hardboard to prevent moisture from further permeating the siding and compromising its structural integrity. Fortunately, you can repair Masonite hardboard siding with wood putty.
Clean the hardboard with a stiff-bristle brush. Scrub the brush around the hole to dislodge dirt and debris.
Pack wood putty into the hole using a trowel. Slightly overlap the putty onto the surrounding hardboard by 1 inch. Completely compress the putty into every depression.
Level the wood putty, using the trowel, to make the repair aesthetically appealing. Leave about 2mm of excess putty for sanding purposes.
Leave the wood putty to dry for the time span indicated on the product's label.
Sand the dried excess putty using medium-grade sandpaper. Make the dried wood putty thoroughly flush and level with the encompassing siding.
Scrub the smoothed putty with the brush to remove sandpaper dust.
Paint the dried putty to match the hardboard siding. Using a paintbrush, apply two coats of exterior house paint over the putty to hide the repair. Let each coat of paint dry individually as instructed by the manufacturer.