Nylon washers aren't needed. And most people don't powdercoat the hardware, if you take your stuff in to the paint shop with the bolts in they would probably blast with them in but take them out to paint and fill the holes with silicone "slugs" as they call it.. If they painted it with the bolts in you would not get them out without ruining the paint job. I am no expert at powder painting am a painter but not a powder painter.. . But I just took all my stuff in to the shop and talked to them a bunch when I dropped my stuff off. I hope my  helps. I'm sure some others here have waaaay more experience than I do with powder paint stuff..

Don't paint your nuts and bolts leave them "natural" whenever I see nuts and bolts with paint on them I think someone lazily painted without properly removing parts. "Spray bombed "  if you were to powder coat nuts and bolts you wouldn't get the proper wrench over them because powdercoating is so thick!!