The 'pro's' work the filler while still soft, so that the cured rough product is almost the required shape and not like your sills. That saves a huge amount of time sanding!

Also, realize that the lifespan of a shining finish is directly related to the grid used for sanding before application of the final coat. I work in the superyacht business where before the final layer everything is sanded (manually) starting with grid 400, followed by 800, 1200 and finally 2000. That's why these boats keep shining 'like the door of a whorehouse'.