Place a sheet of 50-grit sandpaper around a sanding block. Sand the entire bar surface. Switch the sandpaper to 85-grit and repeat this step. Sand from edge to edge on the flat surface, including the lips or trim on the edges of the bar.
Wipe the bar off with lint-free cloths to remove the sanding residue. Pour mineral spirits on a lint-free cloth to dampen it and wipe the bar down. Let the mineral spirits dry for about 10 minutes until completely dry to the touch.
Pry the lid off a can of polyurethane with a flathead screwdriver. Stir the contents thoroughly with a paint stick until it is uniform in consistency.
Dip a foam paint brush into the can of polyurethane and apply it to the bar top starting on one end and working to the opposite end. Use brushstrokes that follow the grain of the wood. Make a single thin coating on the bar top. Use the angled tip of the foam to apply it in corners and along edges. Let the sealant dry for at least six hours.
Place 220-grit sandpaper on a sanding block and lightly sand the coated bar surfaces. Dampen a lint-free cloth with mineral spirits and clean all sanding residue off the bar.
Repeat Steps 3 and 4 to apply a second coat of polyurethane to the bar. Wait 24 hours before light use of the bar or 48 hours until it is used fully.