If you really want to protect your control tubes, consider the INSIDE of the
tubes as well as the outside, since as a practical matter you won't be able
to inspect them when you're finished.

There are several different compounds that can be used to corrosion proof
tube interiors. I used paralketone on the tubes of an ultralight
(Pterodactyl) I built about 20 years ago. It was a messy job. I warmed the
tubes and the paralketone. Then I sealed one end and poured paralketone in
the open end, sloshed the stuff around by rotating the tube and tipping it
end to end, then poured the excess out.  The fumes are nasty so wear a mask.
The tubes still fine, and I live on an island in the Atlantic ocean.

For the exterior of the tubes, having them anodized is ideal but expensive
and not all aluminum alloys accept anodizing.

Ted Stanley - A&P-IA

>From AC43.13-1B:

6-42. TUBE INTERIORS. Protect the interiors of structural steel and aluminum
tubing against corrosion. A small amount of water entrapped in a tube can
corrode entirely through the tube thickness in a short period. Coat the tube
interior by flushing with hot linseed oil, paralketone, or other approved
corrosion inhibitor. The flushing liquid is usually introduced through small
holes drilled in the tubing. Allow the flushing liquid to drain and plug the
holes with a screw or by other means to prevent entry of moisture. Air and
watertight sealing of the tubing will also give adequate protection against
corrosion if the tubing is internally dry before being sealed.