Fiber-cement siding is an attractive alternative for homeowners who want the look of wood siding with the rot resistance, fire resistance and durability of cement. Fiber-cement siding is low-cost for its rated lifespan and is easy to maintain when compared with other siding choices. No matter how durable fiber-cement siding is, though, cracks can still occur. Small cracks can be repaired with a special patching compound before they cause breaks, saving you the work of removing and replacing a damaged panel. Adding a fresh coat of paint over the patched area will blend the repair into the surrounding siding.
Put on a pair of rubber gloves. Mix a solution of 1/2 cup trisodium phosphate and 1 gallon of water. Dip a soft-bristle brush into the solution, then scrub the cracked area of fiber-cement siding to remove dirt and debris. Rinse the siding with clean water to remove the soapy residue.
Mix the latex-modified cementitious patching compound in a small bucket, according to the manufacturer's instructions. Blend the compound until its consistency is stiff enough to hold on the tip of a putty knife.
Scoop up some of the patching compound with a putty knife. Spread the compound over the crack with the edge of the putty knife. Push the compound firmly into the crack and slightly overfill it. Scrape the surface of the crack with the edge of the putty knife to level the compound with the surrounding surface.
Wait three hours for the compound to cure. Keep the surface of the compound moist by spraying it with a mist of water if it begins to dry. Allow the repaired area to sit for 24 hours.
Paint the patched area the same color as the rest of the siding, using a paintbrush. Feather the paint outward to blend the repaired area with the surrounding siding. Allow the paint to dry for 48 hours before touching the surface.