Clay is the way to go. The challenge in automobile design is to have it look good from any angle. This is rarely, if ever accomplished. You have an eye for design for in your critique of the scale model I did you brought up some good points.
Drawings and renderings are very important to car design. They set forth a theme, a personality you want the car to have. It is the clay which transports you off of the two dimensional drawing board and into the world of three dimensions.
Credit cards do work good for smoothing (cutting the clay). What we use in the car biz is spring steel. Check out
www.chavant.com
This is the company which makes the clay used by the auto companies. You will also find pictures of clay tools there also which you can order.
The computer and five axis mill have changed the process of automobile design forever. Having been trianed in clay modeling at the Ford California studio in the 1980's, I learned the old ways of doing things. The designers would draw wonderful rederings of cars by hand and tape them on the wall. Us clay modelers would sculpt what we saw, either in a scale model or full scale. I always enjoyed the full scale.
One regret I had was never working in the wind tunnel. In the tunnel a run would be made on a full scale clay model and then the sculptors (clay modelers) would rush out and change radius's of leading edges or even the slope of a hood in record time so another test could be made of the car with the modifications. The clay modelers I have talked to who have done this kind of work said they were exhausted at the end of the day. Moving that much clay around is very physical and doing it to precise measurments is mentally taxing. But when you see something you worked on pull up beside you at a stoplight or pass you on the freeway at a high rate of speed, it is worth all the work.
Check out the Chavant site and maybe order yourself some tools. If you have a passion for automobile design, which you seem to, it will never leave you. Mastering the clay will open up a whole new world of design to you.
Bondo