C & E wrote:
My is 20 yrs old and the 4x4 supports are sunk directly in the concrete.
Inspection does not reveal any obvious rot... yet. I was wondering if
making a flashing near the concrete/ wood interface but high enough to allow
air circ would be of any use at this late date. I also considered using the
glycol-borax-boric acid mix as a preventative. There is a bit of a gap
between the wood and concrete that should accept this if it's of
sufficiently low viscocity. I know that in the long run I't's a lost cause
but is this a worthy project?

Location of deck? city, state

Is concrete a slab on grade? A pier foundation?

I doubt that the flashing would help much or be worth the effort.

You might consider drilling a hole into the post at a 45 deg angle such
that the bottom of the drilled hole is about 3 or 4" above the
concrete.........

fit the hole with palstic tube & funnel so that your can flood /
saturate the post base with some sort of wood preservative

if you want to go "organic" use raw linseed oil...like the Vikings

do this dance when the wood is as dry as it gets ......no rain or
watering for a long time

cheers
Bob