What use to repair hole in hardie plank siding?

The easiest way to repair a hole in Hardie siding or any even interior walls with sheetrock is to first make a slightly larger hole that is perfectly round. Then make a plug from a scap piece of the same material that is slightly larger. With Hardie panel the plug should be about 1/16" larger and with sheetrock make the plug about 1/8" larger than the hole.

The easiest way to do this is with one of the Hole Pro adjustable hole cutters with their tungsten carbide cutting blades as they make it easy to both make the perfectly round hole at the size that is needed and to also make the oversize plug that you need as well. What you do is allow for the width of the cut made by the blades which is about 1/8" when making the plug.

If you are repairing a large hole (3" or larger) it may be necessary to use a backing strip of wood that bridges the hole and provides support for the plug. The main thing is to have the circumference of the hole and the plug neat and snug. With these repairs I usually get by with a single coat of paint and the patch is virtually invisible to the eye.

The end result is a snug plug that is easy to then paint and you have the same weather and fire proofing characteristics with the plug as you have for the rest of the wall which is important. And outside any temperature induced expansion is not going to disrupt the plug and your repair.