"Cake!" (the Art Car) is still evolving. So far, we've used about 50lbs of Bondo, many gallons of oil-based Floor and Porch Paint and dozens of tubes of caulk. Yes, this stuff is permanent. So far, nothing has flown off while driving on the freeway and the car's MPG is only slightly worse than in its pre-artcar state. We won the Third Place overall "Art Car" award in 2010, and a Second Place "Daily Driver" award in 2009.
You can see pics of the other entries here: http://www.orangeshow.org/artcar.html. If you're not familiar with Houston's Art Car parade, you should know that Cake is amateur hour compared to some of these contraptions. It's worth the clicks to see the pictures.