It doesn't matter if you hand sand or use a machine. Prep work is dirty. First and formost. Degrease and degrease, and degrease as you take a part for prep work. The finer the sand paper the better the finish. 600 grit leaves a nice finish, with a good bite to the surface. Wire wheels are great on rust. Here are some pictures of a 2000, John Deere F-620 Z Trak that I repaired and painted two seasons ago. I used PPG Acrylic Enamle with a urathane hardner. The primers were epoxy primer both as a base primer and reduced as a sealer. The SMC plastic body was repaired and that needed a thicker filler primer also.