Depending on how bad the underside is, or weather or not it has factory undercoating on it should help you decide on which way to go. I used an industrial steam cleaner, the hot steam and pressure blasted the undercoating away. I then went at it with wire brushes and plastic scrapers. Once I was satisfied with the clean up I wiped it down with wax and grease remover.
Personally I didn't want the rubberized under coated look, so I used Eastwood Chassis Black, it has been three years and the underside still looks good.
Good luck with either methods you choose.....and yes you can do it!