On my Spyder project I smoothed the undersides of the rear deck ("clam") and the front lid. Both turned out to be more hours than I planned, and neither is perfect, but I think it will be noticeable whenever I lift the lids in public.
I started out by sanding the mesh and cleaning it with acetone. Then I skimmed on Upol Fibral (like kitty hair but better) very thin to fill the mesh as well as possible, and followed that with 80 grit, acetone and regular body filler, mostly. In the front I actually skimmed the second coat with a product called All Metal which is aluminum mixed with polyester resin. The All Metal finish is enough like bare old aluminum I thought I could maybe get away with not painting it, but that was folly. Both All Metal and Fibral set up very hard and cannot be sanded into shape as easily as regular filler. Use them sparingly.
Anyway, three or four goes with the regular body filler, down to 220, then 2k primer and paint.
(and 550-0051, pre-paint, for comparison):