Bill Harrelson is right.  You can alodine aluminum at home.  Alumiprep 33 is
good, and is a phosporic acid etch.  It also causes the aluminum to foam up
-- it's that effective.  Rinse this off with distilled water (tap water
sometimes leaves a discoloration) and apply Alodine 1201 before the water
dries.  Make sure you rinse off the Alumiprep really thoroughly from the
crevices in the pushrod ends.  It would be bad to have that stuff etching
away for all time.  That's why I do the tubes first.

There are two types of alodine, or more accurately, chromium conversion
coating (Alodine is a trade name).  Alodine 1001 is basically clear, maybe a
slight tint, and allows electrical conduction through the coating.  Alodine
1201 is the darker golden color that we're used to, and is not guaranteed to
conduct electricity, but usually does.  If you're painting over it, I can't
see how that makes any difference.  I use the darker stuff because it looks
better...

Aluminum oxidizes *amazingly* fast.  Knowledgable people have told me that
I've never actually seen bare aluminum -- it oxidizes as fast as it can be
exposed to air, even on a lathe or milling machine.  So that's the reason for
keeping the aluminum wet before brushing, dipping, swabbing on the alodine.  
Keeps the oxygen away.

I think the professionals get better results than I do at home, so I have my
alodining done professionally when I can.  But not always.

FWIW, I had some aluminum painted for a Space Shuttle experiment in the mid
1980's.  The paint shop told me that "the single most important thing you can
do to promote paint adhesion is Alodining" though clearly this applies only
to aluminum.

BTW, phosphoric acid (the stuff that makes the aluminum foam up) is an
ingredient in Coca-Cola.  You can actually take the rust off of steel with
Coke.  I've done it to my ski edges.  Think about that next time you open up
a can of soda.

- Rob Wolf