If you are talking about smoothing out the rough finish on the coats you already have but you don't have a perfectly flat surface I would recommend wet sanding with 600 grit or even finer. I use a foam and rubber block with the paper wrapped around it. The harder side takes the high spots down and the foam side smoothed the lower rough surface. The higher the grit, the slower the work but you have less risk of burning through the paint.
I learned the hard way that it really pays to put the extra time in on the sanding. The finish will last a lot longer.