Question about Alodine 1201

Sorry -- but the cladding is *pure* aluminum! The stuff under the
cladding is the alloy, which is usually somewhat prone to corrosion.

Where was it that I said the clad was anything other than aluminum??

Ok, after I re-read this a bit, I can see where the misunderstanding
might lie. My bad. Misuse of the word "raw", perhaps. But pure alum
rots right fine. The issue is one of porosity (ok, grain structure).
The clad has none (ok, very little) exposed. It's the form, not the
chemistry, that precludes the corrosion.

Although you bring up a good point that different alloys corrode
differently. Some with remarkable ugliness.

This part is true. You should not sand clad aluminum, except to remove
existing corrosion

whew! glad I did OK :-)
Unfortunately, the manufacturing processes involved in aircraft
sometimes leaves the builder/factory with no choice but to do sanding
and lots of it. The clad is routinely removed for coating and MUST be
removed for alodine/iridite, then paint. Of course, these are
"approved" FAA processes, not just what we might do in our own build.
Relying on old chemistry and old processes that never change isn't
necessarily what the homebuilder needs most so turning to the factory
isn't always the greatest thing.

I'm pretty sure there's lots of fine etching epoxies to paint exposed
parts without iridite. But they don't give 'em away.