The better your initial polish is the better it will hold up. Don't let fluids dry over the polish and it shouldn't be an issue keeping it shiny.
My first timing cover was done by one of the shops we use (we have a few different ones that we go to) and it was semi shiny at first. Didn't hold up very well over time. Kaleidoscope polishing in Gardena is one of the better places we use. Can get into the crevices very well, and their high luster polish holds up well over time.
I think there are only a few timing covers fully polished out there that are not rebuilt motors. Off the car it's no big deal to install during the reassembly of the motor.
How many rear covers out there are polished? hahahah, how bout upper and lower oil pans?
