An effective clear anti-corrosion coating for aluminum is class 3 chemfilm,
a.k.a. Alodine 1500, which are all names for a particular chromate conversion
coating. You more often see class 1, which is the familiar gold color. The main
difference is that class 3 is guaranteed to be conductive whereas class 1 is
nominally non-conducctive, though you can scratch your way through it fairly
easily. Both offer excellent anti-corrosion performance, though class 1 is
somewhat better. We always specify the clear, conductive class 3 for enclosures
featuring EMI/RFI gaskets, where 100% shield continuity is required. Bigger
plating shops could probably handle long tubes of a Yagi and the process
normally isn’t too expensive. You can do it yourself on a small scale, too. So
with class 3 chemfilm, you keep your shiny aluminum while maintaining
electrical connections even if you don’t attempt to abrade it away.
While a clear *anodize* would be the ultimate in protection, the last thing I’d
want to be stuck doing is grinding away at it with silicon carbide paper for
many square feet of tubing in order to guarantee large-area connectivity.
Sorry, it’s NOT that easy by manual processes. You could have the parts masked
during plating, however.
As for low observability, that’s my goal here in my CC&R area, so I paint my
verticals. I use NATO non-spectral grey… Well, not the official paint, but grey
primer makes a darned good match to the fuselage of any military aircraft that
I’ve worked around. Non-spectral = flat = no glints in the sun. If I miss a
small spot, it shows up like a beacon in the sun! Light grey has generally
proven best against the “average” sky, thus its wide military use. I prep the
aluminum as follows: 1) buff with fine steel wool, especially if it’s got any
signs of dirt or corrosion 2) degrease with chlorinated solvent 3) wipe down
with Aluminum Prep 79 (available from Aircraft Spruce; it’s basically a
phosphoric acid solution with surfactants; it activates the surface) 3) rinse
with water (you should see few water breaks) and let dry 5) paint with
Rust-Oleum pro primer #7582838. Some of it has been up about 5 years and it’s
good as new upon close inspection. Haven’t been caught by the neighborhood
Nazis. Yet.
Gary, NA6O
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