My cap had been painted silver by the PO, and this paint was mildewed and sun damaged. I sanded it heavily, primed it, and painted it in my garage with the correct Toyota white paint - not a base/clear - just a single stage, and it turned out great. I did not even buff the paint. I put on several coats for wear resistance.
Recall that you cannot see most of an FJ40 roof unless you're on a ladder or in a helicopter. If you get the part that you *can* see looking good (the edges down to the rain gutter), the middle doesn't have to be perfect. If you're looking for a show quality job, you will have to work harder at this than I did, but mine looked really nice. White paint is very forgiving, since it reflects a ton of light, so minor flaws just won't show.
Colo Springs, CO
'78 FJ40, '89 FJ62, '98 4Runner