Mike, I just visited the link you posted with 3D images of your liner. WOW! I like the diffuser at back too. Very interested what Mark, your aero guy, sees air doing under the car to feed that diffuser. It will be a difficult task with a 38 foot long car. Also, at 33" tall (didn't say how wide), your frontal area should be better than reasonable. The top view looks good as well. This is going to be quite a sensation in SA when done (or anywhere, for that matter).
Black paint and sanding blocks; the final answer in finish sanding. Both are extremely effective but I have grown to hate them both. They cost me countless hours sanding, filling, sanding, filling some more. Then there is all the sanding dust that goes everywhere. Still have not cleaned up yet.
John