Burnishing bottom paint involves wet sanding with 320-grit or 400-grit sandpaper, and moving up to 600-grit sandpaper to smooth and polish the finish. A smooth finish creates less resistance in the water, resulting in faster speed.
Hard and copolymer paints can be burnished, but burnishing will not work on soft paints.
For a smooth finish, start by applying the bottom paint with a foam roller or short 1/4" nap roller. Have several foam rollers on hand because the solvents in the paint will cause the foam to become soft and lose its shape. Let the paint cure fully. Wet sand starting with 320 grit, if rough, and move up to 400 grit, then 600 grit. When you are satisfied with the smoothness of the bottom paint (or exhausted), use bronze wool to burnish the paint to a very smooth finish.