On Wed, 28 Sep 2011 08:16:18 -0700 (PDT), JP
wrote:

I have been fighting a losing battle with a screen porch roof that is
made of aluminum panels butted against each other. Sealants on the
seams last about a year (18 months if lucky) before leaks start
again. Each time I clean the old sealants off down to bare metal with
a wire brush. I have tried the normal latex, silicon, polyvinyl etc
but they all fail. The roof gets full Florida sun so it is UV
intensive and hot.

Look into Butyl rubber caulk (or possibly urethane).

"Butyl Rubber® Caulk is a high performance product that is ideal for
sealing exterior gutters, corrugated roofing, pipes, aluminum and
vinyl siding, lap joints, flashing, roof vents and drain spouts.
Provides an insulated, watertight seal for areas exposed to the
elements.

For use on: aluminum/metal, plastics, rubber, concrete, wood, vinyl,
mortar and stone"

http://www.wlcaulk.com/products/rubber_caulks/butyl_rubber_caulk/index.html

--
GO Gators!