Fiber cement siding, like Hardiplank, can develop the typical dents and notches over time. Once you notice a problem, try to patch it as soon as possible. If you have actual Hardiplank installed on your house, or a fiber cement siding product made by another manufacturer, be sure to check with the appropriate manufacturer to ensure that patching the siding will not affect your warranty.
Although Hardiplank and similar fiber cement siding products look like wood, they actually are more like concrete. Therefore, you should purchase a cement based patching compound recommended for use with fiber cement, and not attempt to use wood putty or other patching compounds.
Step 1: Clean the area with a damp cloth or sponge to remove any dirt or other debris from the area.
Step 2: Mix some cement patching compound made for fiber cement, paying particularly close attention to the ratio of mix to water.
Step 3: Apply the cement patching compound to the notch or damaged area of the siding. Shape the cement patching compound so it fits the rest of the profile of the fiber cement siding. Use a plastic putty knife to apply and shape the compound on the Hardiplank.
Step 4: Allow the patches to dry completely, then fill in any patches that have shrunk slightly with another layer of patching compound. Allow the patches to on the fiber cement to dry again.
Step 5: Smooth the surface of the patches with sandpaper. You should only sand long enough to have the patches match the surrounding sections of Hardiplank or your comparable fiber cement siding product. Use a tack cloth to remove the dust created by sanding before proceeding.
Step 6: Paint over any patches so they match the surrounding fiber cement. You may have to paint an entire row of siding to have the patch blend completely.
TIP: If you are patching boards before they are being installed, or have large patches to paint, be sure to prime the fiber cement siding before attempting to paint it.