I use a Mar Hyde product called Tal-Strip. Comes in spray on cans or gallon jug of gel which you just paint on, let it set, scrape it off and wash/brush it off. Two applications gets most of the paint out of the grainy surface.
Then if your going to paint, just paint. If your going to polish, sand the grain down with 80 grit to knock off the high points, and use a dremel to remove the casting flashing (it makes polishing look better).
I used a sanding drum, a flap wheel, a polishing wheel, and a file bit.
Then you get into the polishing aspect. A no brainer kit is available from POR. Their wheel polishing kit is a wee bit pricey but has absolutely everything needed to polish and clearcoat the plenum and valve covers.