John,

I would definitely paint them. I swear by that POR stuff, in case you
haven't noticed. :) Not only can it be painted on top of rust (in fact,
it's preferable to paint it over rust because of the grip), thereby saving
a lot of labor (yes, the loose rust must be brushed off and the parts must
first be rinsed with water and let be dried), but POR-15 also dries to a
very hard surface that does not easily get chipped or scratched. There is
a caveat, however: I'm not sure what nasty chemicals go into that paint.
Michael Heron told me he thinks it contains cyanide derivatives, although
I didn't see any warnings on the can. (Not that it would need FDA
approval...)

I'm not sure which color would be best. I used black because that's what I
had in stock and have been using for chassis parts, but grey may keep the
boxes warmer, I don't know. (perhaps someone who understands the physics
involved can explain to me why color matters for heat conductivity--is 
that the word?--, even in dark places.)

Sami