How to sand exahust

i have no experience in sanding so how should i go about it ? im removing rust and need it flat so i can wd 40 it after

 

Get some medium sand paper an sand the thick rust. An then go over it with a very fine sand paper

 

apparently i should get emery paper

 

Yeah emery cloth is better for metal

 

emery cloth ideally as it last longer but not essential. Wire wool and WD40 clean it up well for the lighter rust