I detail my car engine once about every 2 months. This includes spraying degreaser on the underside of the car hood, then washing off with water. However, I notice this doesn't clean the underside of the hood properly. There is a buildup of dark stains on the metal which can only be removed by polishing.

So, that's what I did. I spent yesterday polishing the underside of the car hood. Not easy because the surface is not uniform, with lots of grooves and undulation. I am not quite happy...