Hi,
I'm more an industrial designer than artist. As such considering the complexity, designing this car is a couple hours work, tops - with the proper software. So, given the unknown quality of the file :
Quote Originally Posted by LambdaFF View Post
Depending on the quality of this model, you're alsmot better off re-designing yourself.
I can't help you fix an STL, I don't really know how to : I make me own files and they work. Have you tried netfabb as suggested ?

Otherwise, I suggest you look for another model in a repository that caters for 3D printing : at least the models are checked for 3D printability.

EDIT : I see he has posted a STEP file. This is a much better quality file that should ensure a proper solid definition. Try to get that and mesh to make a STL : should work better. If you can't I'll give it a look.