fiberglass (old Corvette) cars with great results, used to buy it from a distributor out east and it now looks like you can also get it on amazon.
I use a detailer spray from the same company in-between the polish ... no swirl marks and really kind to the finish without any buildup ... for the 'plastic' or composite panels on the smart I would make sure that whatever product you use has something along these lines ... (from a FAQ list on the mfg site):
Question: Are Race Glaze products safe for use on my boat, camper, or motorcycle?
Answer: Yes, Race Glaze products are safe for use on the following: painted metal, fiberglass, plexiglas, chrome, glass, brass and aluminum; cars, trucks, aircraft, campers, RV's, motorcycles, wave runners, snowmobiles and boats.
As always --- I'm not the distributor or mfg, just passing along a product I've used many times in the past and have been happy with ...