On Friday, March 25, 2016 at 5:35:51 AM UTC-7, N_Cook wrote:
No reception problems , even in heavy rain, but presumably will do some
time.
If the dish completely rusts over ...

Oh, there's a solution for that, from the world of architecture.
Plant ivy.
Seriously, there's very little effect unless the dish changes shape; the
Arecibo telescope doesn't look like a shiny-new dish, and it focuses
radio just fine. And a thin layer of paint won't keep the underlying metal
from blocking the passage of radio waves, so it'll still reflect if you decide
to apply some finish.