Use 36 to get it down to bear metal, then fill n sand primer. At least 3 coats, spray a black mist over the primer ( rattle can black from the dollar store works fine.) Then use 150 or 180 to sand it down, where you have black left, it's a low spot, fill it with body filler. If you sand thur the primer it's high, knock it down with a body hammer. After you are happy with it hit it again with fill n sand primer, guide coat it one more time and hit it with 400. Remember, Never leave primer on bare metal, it acts like a sponge and will hold water and cause rust. You need to put paint over your primer as soon as you can.