Sweep your plywood floor prior to painting it, to remove all dust and debris. Paint will trap any dirt underneath for a messy finish.
Tape down any corners you don't want painted using painters tape. If you have fixtures that meet the floor, such as a sink or cabinet, tape their bottom edge where they meet the plywood floor.
Apply one coat of primer/sealer to your bare plywood floor. To do this, pour primer/sealer into a paint tray. Roll the primer on using a paint roller fitted with a smooth roller cover for smooth plywood or a slightly rough roller cover for slightly rough plywood. Use your small paintbrush to get corners.
Let the primer dry for the recommended dry time, listed on the paint can.
Coat your plywood floor with one to two coats of paint when the primer is dry. Use a paint tray and paint roller fitted with either the smooth or slightly rough roller cover. Let the paint dry in between coats for the recommended dry time, listed on the back of the paint can.
Let the paint dry for two to three days after you've completed the last coat.
Seal the paint into the floor with water-based urethane. Apply three coats of urethane, waiting the recommended wait time in between coats. To do so, fit a paint roller with a lambs wool roller cover. Pour the urethane directly onto the floor in a line, and then push it with the lambs wool roller to distribute it. Work in straight rows, and then touch up the corners with a brush.
Remove the tape when the urethane dries.