1. clean it.
2. best way to paint alloy is to use 400 grit wet and dry paper with hot soapy watar, (this is a must). keep sanding until all areas no longer have a shine. if you have a orbital sander 240 grit dry will be fine. as the primer will fill the scratches.
3. clean again
4. yes you need a primer. with alloy you should use a 2 pack primer but if thats out of your price range than a etch primer will do, or a std acrylic primer but don't use an orbital sander with 240 grit to prepare. now leave to dry. give a light guide coat.
5. once dry wet rub with 600 paper. avoid using your fingers and keep your hand flat on the surface. don't dig in. if you can still see scatches then give another coat of primer and repeat this step.
6. and clean again
7. now paint. persanally I'd be using 2 pack.
hope i helped.