Depending on the depth of the rust, the proper answer is elusive at best. If you don't want the rust to reappear a short time later, all of the rust needs to removed. This may sound like a simplified answer, but how many times have you seen a car at a cruise in and seen bubbles under a nice shiny paint job. My approach is to first treat the spots with a phosphoric acid solution, which is available at any paint supply or home repair store. This will convert rust to harmless powder and will give you better insight as to your level of rust. If you have surface rust, do not use a high grit grinding disk or the scratches will be difficult to cover up down the road. 3M makes a charcoal sponge like grinding wheel that will fit any drill head. It will do the least damage. If you have rust through, you need to remove ALL dead metal. Don't be afraid to pokes holes in suspected metal using a screw driver or awl. If it is soft, it will reappear. Once all of the rust is removed, determine whether or not replacement metal is in order. If it is just surface rust re-treat with the phosphoric acid solution. Any ,exposed to the surface metal, should be covered with a self etching primer. Try not to use rattle can products as they usually react unfavorably with top coats. Any swirl marks or scratches may be covered with " build " primer. Again , no rattle can products. Many coats of concentrated build primer will cover deep scratches. If you are happy with the cover of the build primer, use 320 grit wet/dry sandpaper and wet sand the surface using a sanding block. I prefer wrapping a full sheet around a paint stirring stick. It will wrap around about 3 or 4 times and allow you to peel off fresh surfaces of the paper as it wears out. If you use finer than 320 grit, the top paint might not adhere. Any less than 320 and scratches will be obvious. You have most likely heard that a good paint job is 95 % prep. Believe it. If you can see flaws, it is not nearly ready to paint. If you can feel them it is not ready. Have fun