I need to replace a broken section of exhaust. Should I replace the gaskets also? one of the gaskets is directly between the engine and the new part. The other is between the new and an old part of the exhaust.
You should replace any gaskets that will be disturbed by replacing the pipes. Clean any old gasket material off the surface that the gasket sat on with a putty knife or something similar. You want a nice clean surface for the new gasket to seal against. After everything is reassembled start the engine and place your hand near the pipe joints to feel for leaks.
gasket surface prep is an art and a science. yes, always replace gaskets when adjacent parts are disturbed. it is important to have a clean, even, sealable mating surface on each part so that the gasket can do it's job. different materials and shapes will require different approaches. i would avoid using a putty knife in general on aluminum due to the risk of scratching the mating surface. harder metals like cast iron and steel are more resistant to scratching, so a putty knife can be ok here, but be careful not to gouge. sandpaper can be used, but only on a flat surface with a sanding block, and again, be careful not to gouge or create low spots, and you want to use decrease the grain size pass after pass to get a smoother surface. make sure debris is not left to get trapped in your catalytic converter. clean the area thoroughly. a good seal on your exhaust system is important to avoid carbon monoxide poising of the driver leading to a fatal collision. if you can't get the gasket mating surfaces flat, clean, and smooth, have the parts machined by a machine shop for a few bucks.