There are many ways to achieve an effective car paint simulation in 3D. Several companies have written high end shaders which give amazing results but most (not all) of them require fine tuning of a lot of settings to acheive a quality end result. Using Softimage XSI and a simple Phong shader combined with some basic render tree setup and the Mental Ray render engine will give professional results comparable to using a 3rd party car shader.
LINK:
http://www.cgarena.com/freestuff/tutorials/xsi/carpaint/index.html