take something flat, piece of steel or file...and 80 -120 sandpaper and randomly sand it , each time you pass over it you will incur a low spot (minute) but over time these low and high spots will "cancel" each other out and give you a pretty good flat surface, as to using exhaust sealer (brownish tan stuff) don't think it would hurt at that connection aft and not pre cat. ,however properly and sequentially torqued gasket will do it's job, that is why there are only three bolts rather than a greater number.