Mark,
  When I built our IV-P I went thru a process I learned working in the metal
fab shops in third party maintenance facilities for large aircraft.  It goes
like this:
  First clean and polish the tubes.  I use a fine scotch brite pad or fine
aluminum oxide paper of 400 grit.  This will take out small scratches but
will not damage the surface.  It is possible to do the inside but that is a
lot of work.
  Next I use aluma-prep, PPG aluminum cleaner DX 533.  It is phosphoric
acid.  Wet a piece of towel with it and scrub untill water sheets off, no
beads.  Rinse and dry.  Then apply alodyne, PPG aluminum conditioner DX 503,
with a clean piece of towel untill the metal is golden color.  Rinse and
dry.  Assemble the tubes at this point.
  The primer I use is a two part self etching zinc chromate, DuPont 625S.
Following that I top coat with a good polyurethane white paint.  There you
have it, pieces fit for a transport category aircraft.  Corrosion protection
I estimate good for fifteen years.
Walter Dodson  IV-P  18.5 hrs