i did mine and either way takes a while, but i found rubbing back the plastic a bit first, to get the worst of the dimples out notonly is quicker it'll save you in filler primer as it doesn't go too far, being so think. i've done my whole lot and done a few bumpers and as long as you take your time it should be fine.
as with all paint work, there are no short cuts so don't go looking for them!! the preperation will pay off when you come to paint them.