Larry, I build the clouds in vector first, then I use a digital brush to finish them off. Just seems to work best for what I want. I used 6 different layers for these clouds so far.
Gary, yes I do create a 3D wire frame first to work from. It allows me to be creative as to the subject and composition I want to use. I do some real odd ball machines at times, which can have little or no photo examples to work from, such as this German plane.