Spraying with rattle cans is easy as long as the thing you're spraying is horizontal. To get a good finish on it, you need to be liberal with the amount of paint you spray on it. If you're spraying on something vertical, it'll drip.
Sand it down (it comes off quite easily), clean it up (you may want to use metal filler for any small cracks), and stick about half a tin of spray primer on it. The important thing now is to let it dry, leave it overnight in a warm place, the longer the better. If you spray when it's still drying, the paint will crack and give you a crazy paving effect on it.
The fun part is just whacking loads and loads of rattle can paint on. Put quite a thick first few coats on, and then thin the layers out, making sure it's touch dry between each one.
I did my alloy and my rocker cover and they look immaculate. I've been spraying for about five years though.
"C20 LET bang"