AAAARRRGH! No nylon bristles. Sponges are not great either for that matter. Unless it is a very soft Boar's hair brush I've not seen a brush that will not leave swirls or otherwise cause problems. If you are doing the undercairage or tires that is one thing but please keep synthetic bristles off the paint. Try using a 100% cotton chenile mitt. The nap should get in crevices nicely and it will be softer than just about anything else on the finish.

Hope this helps...Lee