- The standard way to "thicken" in 3DS Max is to use the
Shell modifier . Thicken the model a safe distance suitable for the material you're printing in.
- Otherwise, as you can see from your attached JPEG, the model is kind of a mess. If it looks fine in 3DS Max then that means you had an exporting problem, or that Shapeways couldn't manage to import it decently, which is also your problem. Fix the following modeling problems and then try exporting into various formats until you get one that works.
- Your vertices might not be properly welded, which might be the reason it imported badly, and might be why the Shapeways rep told you your polygons are "not well connected". Look up welding in 3DS Max.
- If your wheels are meant to actually spin, you need to intentionally design it that way using the right clearances and mechanical parts. Tutorials on the Shapeways site can help with that. If the wheels can be fixed, then you need to model some connections to the body of the car.
Good luck