My "OEM Grade" MagnaFlow cat broke at one of the welds (cat/pipe joint nearest turbo) after 5 years so I'm replacing it. (BTW, my original cat lasted ~11 years so you be the judge.) Just got the old unit out. Last shop apparently used some copper based gasket sealer on the turbo to cat flange joint. Do I need to use this on the new gasket?
Also, the flange on the downpipe appears to have some remnants of the prior gasket literally welded to the face, see the image below. (Turbo flange is perfectly clean/smooth.) I tried to scrape it off but no success. Is this going to be a problem for making a good seal? Or use the sealent here, too?
I've R&R'd my cat/downpipe twice over the last year. Just used new gaskets and all is leak free. My recommendation is not to use anything additional. Just get that surface clean/smoother with some emory cloth or something.