I used to work at a powdercoating shop and you can coat them with the spokes on. But if your wheel ever gets slightly bent or whatever it’ll be hard to true the wheel cause the nipples are so coated in paint! I would do it. Just make sure when you drop off the wheels, you tell them what to tape off on the hubs! And make sure there’s no grease or it’ll leak all over in the over after it’s been coated.