I hope this will help you somewhat Also a tip from me! When you are starting and learning how to brush, it is very important to just brush small stuff like Bumpers and such.. Just to find your brushing style. Also try to C/P in bumpers on the car then rebrush them, that will help you alot! Now Good luck!
Here's a resource that will give you all of the tutorials you could ever want on brushing...just look through the pages and find some you like. Even if they have nothing to do with cars you will learn a lot. http://www.tutorialized.com/tutorials/Photoshop/Brushes/1