Thanks!
I have a whole polishing kit with different compounds and polishing wheels which I used. Its professional polishing equipment that I've been using for a while now.
The tight spots will need to be done by hand or with a dremel and different tips (felt, steel brush, etc).
The process for polishing depends on a few criteria, like the type of metal (aluminium vs steel), the condition of the metal (rough, pitted, smooth). Most importantly it takes a lot of patience. The valve cover took about 12 hours of work but looks really nice now.