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James HardieÂ recommend using caulks that remain permanently flexible. I think GE's Silicone II, available, well, everywhere, is a good one for the purpose.
GE exuberates about its outdoor use:
<q><strong>Defy the Harsh Effects of Mother Nature: Sun-/Freeze-Proof</strong>
The outside of a home is exposed to harsh outdoor elements throughout
the year. Extreme heat, the sun's UV rays, and freezing temperatures can
cause acrylic caulk to lose its flexibility and degrade. The GE Silicone II*
permanent flexibility keeps it from breaking down so it can protect a
home as it settles or as the seasons change and joints expand and
contract. This movement can cause degraded acrylic caulk to crack and
crumble over time, leaving gaps for air and water to seep through. Those
leaks can lead to water damage, mold growth, and higher energy bills.
Anyway, good brand, good product. 5 year mold-free. I would go with that.