Thankyou for your suggestions. I have previously got as far as TIG's solid example, but creating a thin shell is what I've been unable to do.

My apologies for not explaing very well. I would like to produce a model bus similar to the one below.The curves at the rear of the bus are what I need help with.The bus needs to have walls approx 1mm thick in order to be printed on 3d printer. The curves are of different radii. On the example below some of the curves are not very well curved.

http://sketchup.google.com/3dwarehouse/details?mid=1dea3af9695415f71f6a5b0146bf3030&prevstart=0

How can I construct a twin walled model with curves in 3 planes ?