I remember reading about very substantial increases in lifetime for
tubes shrouded in black, heat-radiating shields.

I have an early series R-390A, I don't care about keeping it pristine,
and I want to introduce all the official service mods, + a few more
aimed at improving reliability.

Its tube shields are all unpainted. I am tempted to paint them black,
using engine type heat resistant paint, but there are some unknowns.
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- Are "proper" black shields so due to black-anodization treatment or a
paint layer?
Are they blackened BOTH inside and outside?

- Is engine paint appropriate?
(Most types are said to improve heat radiation).

- Should one paint shields both outside and inside sauf the electrical
contact area?
(I could e.g theorize that on the inside all transmission is by direct
heat transfer through glass-metal and air-metal contact, not going to
improve with a paint layer).
Are "proper" shields blackened both inside and outside?

- Would the same reasoning hold for at least the _top_ dustcover of a
radio?
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To do a proper job I should first check for temperature changes at the
tube surface in the bare/bare, bare inside / painted outside, and
painted/painted cases, but I am sure there is some knowledge floating
around about this, enough to skip the experimental bit.

Hints, o brethren?