Remove exhaust donut gasket residue?

I'm trying to replace the donut gasket on our 2008 Pontiac Vibe
(rebadged Toyota Matrix) -- the donut at the connection between the pipe
connected to the resonator and the pipe connected to the muffler.
I've scraped and chipped away a lot of black and grey stuff, but there
are still lumps in places, and in other places something with a regular
cross-hatched appearance that looks as though it might have been the
outermost layer of the original donut.
Do I have to get down to bare shiny metal? If so, is there an an
alternative to just chipping away and wire-brushing or using a
small-enough drum sander?
The muffler and pipe are off the vehicle, so I could even use a propane
torch if it would help.
Reply to
Percival P. Cassidy

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