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.
Perce
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