Depends on the part..
If old rusted bolts no grease or oil on them, I use a mix of phosphoric acid and water. If greased or oily, I use paint thinner or mineral spirits first, then derust.
If large parts rusted, I made a derusting bucket with rebar anodes and washing soda and water. Hook up to a battery charger and let it do its work.
As far as paint, I use cold galvanize rattle can and or zinc chromate primer then top coat after thorough degrease and dry. I put my cleaned parts in oven for an hour at 300 to dry.
As far as topcoat, different parts, different glosses of black. depends on what you feel appropriate at the time. You can always correct it later.
