Before the part is coated, it must be cleaned of oil
Before the part is coated, it must be cleaned of oil, dirt and rust. This can be done chemically, but many small custom powder coating shops will wash the part and then "blast" it with an abrasive grit. Sand-blasting used to be common but many shops are going to better blasting materials that work faster. For delicate parts where detail is important, you might want to have the parts glassbead-blasted before taking it to the powder-coating shop, as glass beads will clean the part without eroding much of the detail finish.
With the part blasted, some shops use a chemical etch to provide extra bond to the metal. Other shops move right on to applying the powder coating. I haven't seen a big difference in the quality of work with either method, but if the parts are going to see a lot of service in a harsh environment, the chemical etch might provide a slight advantage.
Another difference I see in some shops is that some will spray the powder on a cold part, while others heat the metal in the oven before spraying the powder on. There is an advantage to preheating the metal, as it enables a thicker layer of powder to be applied to the part, but there is the extra cost of running the oven longer.
