I've been doing a lot of this lately an it does take one hell of a long time
I use about 180 grain wet an dry to get the worst off or paint, then go 300ish
After that it should look like brushed alloy, bit nice an bright
next you can get polishing kits from ebay for about £22ish, these kits connect to an electric drill
You get a spindle that fits both wheels, the wheels are one hard brush wheel and one cotton wheel,
first put the hard brush on, they also come with polishing wax, brown wax is for the hard wheel, load the wheel by spinning the wheel on the wax bar, or by rubbing it on the area your doing
The key to polishing alloy is don't over heat the metal or it will get a ripple effect, once your happy with the surface i.e. no pits or ripples, then change the wheel to the cotton one and repeat it all again with the cotton wheel and the blue wax bar this will bring it up to like a chrome finish
finally a quick going over with autosol and it will look amazing, if its an interior part you only need to polish it up every so often, if its an external part it will need laquer, if when you laquer it sometimes you loose the shine
One way around this is to warm all the area as much as you can with a hair dryer first, then spray a very light coat of laquer and repeat warm then spray until your happy with the effect
Its a real art though so take your time and the speed of doing it comes with time, its a lot like body work but worse to do in some cases as there isn't any paint to cover iffy bits