Try smoothing with a hot airgun. Works great for me. It takes a lot of time and patience but the finish is very nice and shiny and is way faster and a lot less messy than sanding. Don't overheat it though because repairing is nearly impossible so take your time, it's a slow process. I do recommend at least 25% or even 30% infill for this method because you dont want the shell to start "hanging" between the infill. Try on a simple print at first and the go bigger.