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I tried to etch and alodine an already anodized part and it didn't work.  The
part in question was an aluminum piano hinge -- I hadn't realized that it was
already clear anodized.  The only place the etching and chromium conversion
(that's alodine, folks) occured was where I had drilled holes through it.  
The hole edges are now golden.  What a waste of time.  If the anodize coating
protected the aluminum against the phosphoric acid etching, it should be
plenty good in operational service.  

This is why I state that anodize is a perfectly adequate anti-corrosion
coating as far as I'm concerned.  Painting is not required, but if you must
paint it for aesthetic reasons, just clean it thoroughly (acetone or MC) and
paint directly on it.  Be aware that many colors are available for anodize,
such as gold, red, black, clear, and probably green as well.

Having said that, I had the opportunity to have some large aluminum pieces
professionally painted with a polyurethane paint prior to flying them on the
Space Shuttle in 1985.  The paint shop told me that alodine (more accurately,
a MIL-C-5541C chromium conversion coating, of which alodine is merely one
trade name) is the single most important thing you can do for promoting paint
adhesion to aluminum.  However, scraping off an anodize coating to get the
alodine would be pointless, IMHO.

- Rob Wolf
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Please send your photos and drawings to marvkaye@olsusa.com.
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