Why use a product on an application 'clearly' outside its design
parameters.  Just use the sealant that *is* designed for exhaust
temps.  Usually called "Muffler Seal" or somesuch.  Calcium Silicate
IIRC.  Get a bit of fiberglas screen and wrap the affected
discountinuity.  Plaster on above mentioned sealant.  Wait 24 hrs.
Start engine. End of problem for the next six months or so. Rinse, repeat...

Works for me!

Jim Arnott
WetWesties
Union, OR