Hi Kirk!
I had the same problem and significant rust had developed before I
discovered it!
Here's what I did:
 - Firstly, you must (MUST!) remove ALL rust at the tower/concrete
interface.  I used a star drill to dig into the concrete until nice shiny
galvanized tower was visible all around all three legs.  Then I (actually
KF3P) sandblasted the legs until nice shiny steel showed thru every place
where rust was previously present.
 - Next, I used cold galvanize paint to coat the bare shiny steel, then a
second coat the next day.
 - Next, I applied a Rustoleum primer intended for galvanized steel  (I
understand they have changed the name, but the paint and primer were
then known as Galva-Grip).  I then applied the finish coat of
Galva-Grip paint (not primer).  I also allowed the primer and paint to
coat the nearby concrete foundation so that it created a seal around the
interface.
 - Lastly, I used "GE Silicone II" Concrete seal to thoroughly seal the
concrete/steel interface from the weather.  It adheres very well to
concrete and steel.  
Like you, I considered using a layer of concrete, but
rejected that approach for the same reasons u did!
Be sure you re-grade the area around the tower base so mud can never 
collect on the tower foundation again!  And inspect the foundation
annually...
Good luck!
73
Frank
W3LPL
donovanf@sgate.com   
 
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