Smooth parts definitely don't use the brush! Will make fine scratches. On the covers I use a cloth and polish, then buff with a cotton wheel in an electric drill
This polishing above doesn't work on the sandcast parts though. The dimples get black. Use the brush without any polish. Gets into the dimples and casting lines. The crankcase was quite dull before. Removes stubborn marks too.
Make sure it's pure brass bristles, not brass-coated steel ones.
Lordmac cuts his lengthwise to get into cylinder fins. I've just done outer edges so far.
It does leave a slight brassy tone. So I'm the Man with the Golden Engine. (Raised eyebrow.)