Ralph sez:
<<Seems to me that what you are trying to do in essence is to weatherproof a
screened porch exposed to marine conditions, without reducing the vapor
transmission of the voids in the screen; and that this may -- just may -- be
somewhat self defeating, >>

Perhaps not mutually exclusive.

The following demonstration of water-repelling capability of Thompson's
Waterseal water-repellent was described to me by the chemist who designed a
round of refinements to this product in the early 1990s. Take an 18" square
of a single layer of cheese cloth. Dip the cheese cloth in the
water-repellent. Allow to dry. Lay the cloth on a flat surface and gather up
all four corners forming a pocket. Slowly pur 2 cups of water into the
pocket. Ta-daaaaaaaaa. The water does not drain out through the openings of
the cheese cloth. It is held in place by a combination of the surface
tension of the water and the "static pressure" created by the
water-repellant oils on each strand of the cloth. Next pour out the water
and blow through the cloth -- air still moves freely through it. I performed
this demonstration myself, it did work as described.

(NB: I have never used Thompson's Waterseal on porch screens. Your mileage
may vary. This exposition does not imply an endorsement of any product that
may be mentioned. Dis-regard any facts presented or implications made here.
John Leeke will not assume any responsibility for how well you may or may
not have learned common sence from your mommy.)

John Leeke