If you coat it on the outside it won't rust but you might chip it and it
will be very pretty except where it is chipped.

If you coat it on the inside, that portion of the exhaust heat that normally
dissipates through the manifold will go on to the headpipe and it will
run hotter than normal. Since it is much thinner than the manifold you may
shorten its life. It might give freer flow for a while but it will get dirty
before long, just like your exhaust ports so you probably will only have a
temporary gain if any.
--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma