Well its the same stuff that goes on your tires, how long do they stay clean and shiny being on the road with dust and dirt v.s in a closed hood environment, cause after you shine the wheels up they don't form dust rings on the tires right after
BTW the good stuff I used lasts way longer than the turtle wax foam on tires. Also I did not cover anything in my engine bay, I just didn't spray it directly on the alternator, belt, pulley groves etc. But I sprayed it everywhere else that you mentioned, sprayed it all, foamed up real good too, foam on top of foam