the way i do it greats wonderful results but is labor intensive. you need to take apart each major assembly ie. fan case, switch housing, brushroll head, handle fork and sani emtor and have everything removed that's trim or bag related or any plastic.
i start by using a vibrating hand sander to get gouges out on various parts. the on the parts that have had the vibrating sander on them, start with 320 grit sandpaper using the wetsanding method with a combination of mineral spirits and mothers mix as the "wet" part. i change directions as grits get finer (up and down then left to right etc) and increase grit size to 400, then 600,1000 and 2000. if you're just removing oxidation starting at 600 or 1000 grit will suffice. after then i use 0 then 0000 steel wool with the mothers/mineral spirits mix in a clockwise and anticlockwise pattern all over the aluminum. (wipe off after each wet sand grit with microfiber cloth, this method is very messy)
then off to the buffing wheel. using a cleaning rouge first, then a polishing rouge. (make sure to get vice grips and run the tips of screw heads on the body, and the handle fork pin as well, since these arent on the vac)
this will ensure amazing results, but does take an entire day to do.
id suggest the sanding method if your machine is dull and has no more "shine" at all to it. only mothers/steel wool is good for kirby's that havent gotten that badly corroded and still have SOME shine left. it takes less time, but a good buffing wheel set for a bench grinder is a must in my book for "hard luck case" machines.
also while you're metal finishing, a bucket of simple green concentrate and HOT water to soak trim and plastic parts in to clean gunk off is a good idea too.
after your done use meguire's tech 2.0 wax to keep your work from corroding. and a microfiber cloth get get the wax fogging off, and a fine terry cloth to buff with after. this will ensure that is the LAST time you have to run your kirby thru a buffing wheel.
