It's easy mate. If I can do it anyone can. Like said a wire brush (like pictured above is needed. I brushed each one for about 10-15 minutes. Be as hard as you like with the brush they are sturdy parts. It all depends how long you have got to do them. If you want them all done in a day (like me) Then just paint them after wire brushing. If you can afford to do one a day, then go mad with prep work, use brake cleaner sand paper and what have you but bear in mind you need this to dry before you can paint or the paint wont stick well and the finish will be poor. You need to mask up the whole brake disc, including the outside edges and you'll need some thin (about 1cm) strips of paper to put along the top of the disc where it goes behind the calliper (this will become clear when you actually do it). You also would be best covering the arches with newspaper and the surrounding pannels of the cars body. I didn't bother with the latter, you do get bits of overspray that will drift about and land on random parts of your car but they will come off after your first wash and if not, definately will with clay.
My tip would be to use the hammerite smooth spray (it's designed to be applied straight to rusty metal). It's quicker drying, meaning you can do more coats quicker (I did 4 coats on mine, to be on the safe side!) You also, in my opinion, get a better finish with the spray having seen some done by hand.
Another tip is while the wheel is off it gives you a good opportunity to give the arches and that a good clean, and also the inside of the wheels. If I was you I'd clean the wheels while waiting for coats to dry. Also if you can use a trolley jack it's obviously much better and safer and remember to loosen the nuts off both wheels before you jack up and then you can do two wheels at once saving time.
Here's my results.
