Fiber Cement Siding Repair

I have a hole at the end of one of the fiber-cement siding planks on the exterior of my house. The hole looks as though someone took a bite out of the end of the plank, exposing the weather barrier. The plank in question butts up to the frame/side sill of a window. The hole is a bit smaller than the size of a silver dollar. Is there a simple way to repair this or is it necessary to replace the whole plank? Perhaps some type of fiber-cement patch or putty to fill the hole?
Also, what is the best type of caulk to use to fill in the gaps where the siding meets the wooden window frame/sill? DIY Help Pages that I found on the Internet mention ASTM C920 caulk- either acrylic, latex, or urethane. Does one type perform/last better than others?
Thanks in advance.
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Concrete patch would probably work. Even drywall mud (the "add water" type, NOT premix) would for a hole that small. Roughen up the inside of the hole.

These days they're all about the same - quite long, tho, urethanes will probably stand up best long term.
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Since you mention putty, try Bondo.

Something exterior, climate friendly - pick your poison.
Oren They have computers, and they may have other weapons of mass destruction. Janet Reno, Attorney General, Friday, February 27, 1998
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What's a "silver dollar"?

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