Combine 1/3 cup trisodium phosphate with 1 gallon of water in a bucket. Dip a
scrub brush in the mixture and scrub down the rafters. Allow them to air-dry.
Run an electric sander, outfitted with a piece of 220-grit sandpaper, along the rafters. Continue until you've sanded the entire surface area. Wipe off the dust with a tack cloth.
Apply painter's tape to any areas you don't want to coat with finish. Spray a light coat of tinted polyurethane over all the rafters. Allow the coat to dry overnight. Apply a second coat, allowing it to dry overnight.
Screw in 2-inch-wide cup hooks from which to hang lanterns onto select rafters. These should be in areas where you won't hit your head on a dangling lantern.
Place a decorative lantern on each hook.