Coating over rust, grease or old paint fails - it looks fine leaving the shop and peels within a year. Every part is stripped, degreased and, on most metals, chemically pre-treated before it ever sees colour. If a part still has old paint or heavy rust on it, it gets media blasted first, either in-house or by a blaster, before we touch it.
The part has to survive the oven, which rules out most plastics, anything with rubber seals, bearings, electronics or fluid still inside, and wood. The oven also has physical dimensions, so its size caps what we can take - that is the first thing to check before you bring in something large.
Every part hangs from a rack hook to go through the oven, which leaves a small contact point where no powder reaches. We agree with you ahead of time where that point sits, especially on a visible piece.
