WHOA! that is some nasty gouges in that rocker cover & some not as bad ones in the bracket, As mentioned I would just (Save a lot of time & effort) paint most bits & polish whatever you can that is nice to start with
This is one of the other reasons I dont like using polishing wheels on a grinder or a drill, One little slip, Put a nice gouge in it & the piece is stuffed
+ I see a lot of people that use a wheel to polish end up distorting the original shape of the part, The flats have ripples & the edges become distorted & more rounded in someplaces, Basically making it look shit, For example, With the bracket shown above I would actually use a flat rubber sanding block on the face & flat edges & get the surface as FLAT & smooth as possible, Exactly like you said it is about 95% preparation, You spend most of the time smoothing & flattening the surface going down the grades before you actually get it shiny
Thats the way I see it but I am a fussy prick that hates seeing shit jobs like 5 min polishing that may be a bit shiny but the surface is all distorted & crap
"You cant polish a turd but you can roll it in glitter"
Here is some other polishing that I have done