Feb 20, 2015 · Surface finish specification and comparison. Finish Symbol N ... 1.6 N8 125 3.2 Medium Machining N9 250 6.3 ... The Hoover carpet cleaner makes quick work of cleaning carpets and upholstery. To keep your Hoover carpet cleaner operating efficiently, you should clean the machine at least once a month depending upon how much you use it. Cleaning the Hoover carpet cleaner requires no tools because the parts snap on and off.