You  might have a compatability problem. Some Clears are made to go over a
Basecoat that is compatible. There  are some non-catalytic clears that dry
by flashing off the solvents, but you might find that because of the high
shine the clear  might seperate over a short period of time.

BG

Last year I used various engine enamels to "tole-paint" the alpine scene
cast into the doors of my Timberline wood stove.  Hard to guess the back &
front temps of the plates during combustion, but stack temp at the heat
exchanger varies from 200 to 400 F, so I guess 600 on the inside of the 1"
doors max.  Black stays black but loses gloss, GMC green holds up OK, white
yellows a little, and F**d blue slowly & mellowly goes to hell.  All stay on
OK.  My big pisser was that it looked so nice new that I oversprayed the
whole project with FLAPS hi-temp clear, and the clear all browned, even over
other colors that were holding up nicely.  I had to retouch the whole thing.
Doesn't look too bad now.

"High" is sort of relative.  Your metal is thinner, so should be close to
the indicated water temp inside, but heats & cools quicker than iron.
Easily available spray clear is no good at the temperatures I see (or on an
engine block IIRC).  POR-15 has a clear, and my recent experience with their
products is real good.  I'd look into that.