the first sealer one should use in my opinion is a 4" grinder either with
coarse paper or a grinding wheel on it to get rid of all the rust and junk
and bring the flanges back into line.  then 90 percent of the time all
you need is the gaskets.  beyond that i have used some "red" sensor safe
exhaust rated rtv to make up small imperfections that didn't seem to want
to seal.

jonathan