Rod,
The surface treatment will vary somewhat depending on the metal used in
the carburetor body. Most carburetor bodies that would pertain to this
mailing list are made of die-cast zinc alloys.
The yellow, yellow-green, green, or olive-drab patina on most zinc
alloy carburetor bodies is the result of a dichromate or chromate
passivation/conversion coating. Many shops that used to do this have
stopped, since the safe disposal of the highly toxic effluent makes the
process more expensive. However, some carburetor or plating shops still
do dichromate/chromate work. I'd call some plating shops in your area
and see what they can do for you. TIP: Don't let them bead blast the
carb bodies. Even plastic media or high-pressure water can peel flakes
off of the surface. If the alloy is highly homogeneous, blasting is OK,
but MANY die-cast zinc carb bodies are anything but homogeneous. I've
seen flakes the size of a nickel peel off after bead blasting or
pressure washing. Have them chemically clean the bodies before
treatment instead of blasting them.
I've heard that there are some molybdate-based passivation processes
that work well as a substitute for chromate-based processes. There is
also an enhanced chromate coating that uses a mixture of chromating
components and an acid-based polymer. It is said to improve corrosion
resistance while reducing toxic effluent. There is even a black zinc
coating that consists of a green chromate conversion coating that has
been blackened with dispersed silver particles.
There are also dyes that can be applied to an etched surface to color
it, though I've heard that they don't provide anywhere near the same
measure of corrosion protection or longevity as the zinc dichromate
finish.
I've seen spray-on fuel-resistant translucent paint coatings, but I
have no idea how durable they are. Possibly if the surface is prepared
with an etchant or clear conversion coating, a thin layer of colored
translucent enamel might hold up pretty well.
The MIL-C-17711B spec covers conversion coatings, but I haven't read
it.
I'm going to be taking some carburetor bodies in for some experimental
conversion coatings in about a week. I'll post the results to the list.
Regards,
Alan
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