MographPlus has a nice tutorial on using the Car Paint Shader in Arnold for 3ds Max, to create realistic car paint materials. It even comes with free project files so you can follow along.
Download the free project files from MographPlus here.
Please note that this is a more advanced tutorial and is designed for users with a solid foundation in Arnold for 3ds Max.
Arnold is an advanced Monte Carlo ray tracing renderer built for the demands of feature-length animation and visual effects. Originally co-developed with Sony Pictures Imageworks and now their main renderer, Arnold is used at over 300 studios worldwide including ILM, Framestore, MPC, The Mill and Digic Pictures.