[Richard Hudgins]

Machiavelli.

The stainless being stainless, (depending on the alloy) would not rust on the inside.

Therefore you could certainly just coat the outside.

The side-pipe design with the dead air space should work fairly well as the thermal conductivity of Air is 0.024 k - (W/mK). However where the outer skin is welded to the inner tube you are going to get significant heat transfer.

In other words hot spots at those locations.