I went through this but in the end I wasn't able to get a good clear coat on it to keep it from rusting. Aluminum is easy but steel rusts so quickly especially when it gets hot. I tried to coat it with POR-15's "Glisten PC" clear coat that is designed for clearcoating polished steel and other metals. But it was very difficult to apply. I think it needed to be thinned much more than they suggested and then it would need multiple coats. Don't expect to spray a rattle can clear on polished steel because it won't hold up. It has to be an absolutely air/watertight seal otherwise the cancer will get in and it will spread.

Here's mine in the process of polishing: pic #1 I went through several grits of sandpaper by hand up to I think 1000, then 2 different buffing compounds down to a very fine grit, then polished with POR-15's "Artisan" polish (very very good polish by the way - highly recommended!!!)

Here's a pic on the engine: pic #2 You can't see it here but it is already starting to rust under the clearcoat. But now I have a spare valve cover and if I ever get time I'll probably just clean it up and paint it :) In my opinion it's not worth the time and effort because of the difficulty keeping the rust away.

Good luck.
Michael