HardiePlank is a cement lap siding product used on commercial and residential exteriors and is designed for very low maintenance and a long useful life. Though it is a tough wearing product, the surface of the siding can sometimes be damaged during installation, by unusual wear or by impact. The James Hardie company, makers of HardiePlank, recommends the use of a cement patch product made of an acrylic mortar compound for patching their siding products. Acrylic mortar compound can be found at most home improvement retailers and hardware stores under various brand and trade names. The mortar will bond permanently with the siding giving a finish texture close to the original, helping it to blend into the surrounding siding.
Brush away any chipped paint, small loose pieces of siding or anything else that mars the plane of the siding.
Sand over the damaged area lightly but thoroughly with a medium-grit sandpaper to smooth any jagged edges and create an easy grip surface for the patch product.
Apply the patch product to the damaged area with the flexible putty knife to fill in any indentations or holes.
Hold the knife roughly perpendicular to the siding patch to finish the surface of the patch, and taper out the edges of the mortar patch blending it into the surrounding siding.
Allow the patch material to dry thoroughly according to the recommendations on the product label.
Sand the dried patch material lightly with a fine-grit sandpaper to smooth out the surface and blend the edges of the patch with the surrounding siding. Feel along the patch to identify any bumps that need to be smoothed out with further sanding.
Wipe or brush away any of the sanding grit with a clean, dry, lint-free rag or a clean and dry brush, leaving a smooth surface.
Repaint the patched area as soon as possible to protect the patch material and restore a finished appearance to the siding. Use a high quality, 100 percent acrylic exterior paint that matches the rest of the siding.