Sand the baseball bat with the electric sander until it is smooth. If you are using a bat that is already finished, make sure you remove all of the existing paint or varnish.
Coat the bat with one layer of the polyurethane spray. Hold the can around 12 inches away from the bat while spraying and try to coat it evenly. Allow the polyurethane to dry, which will take around 30 minutes.
Rub the bat lightly in a circular motion with the steel wool. Make sure you don't completely remove any of the finish; you just want to smooth it out for the next coat.
Apply another coat of the polyurethane spray and allow it to dry for 30 minutes. This will be your final coat.
Sand the bat again with the steel wool to give it a smooth, even finish.