I just looked at a project in Nantucket with 1847 flushboard siding.
There is both vertical and horizontal siding, where, as expected, the
joints have been moving.  I thought traditionally the joints were filled
with oakum and putty, where there is still evidence of putty. The
previous Durum wood filler has failed, and the caulk on all horizontal
joints, from top to bottom, have accelerated the 4 year paint job's
failure.  The specifications call for using Phenolic Resin caulk. One of
the stated causes for failure was using an organic based caulk, causing
mildew to form.  Understanding that the siding needs to breath, and
sealing in a joint can trap moisture, does a caulk become the best
method?

Curious in RI

Rob Cagnetta