Brush down the fireplace with a fireplace brush, loosening all lodged in soot and dust.
Pour 1/3 cup trisodium phosphate and 1 gallon water into a bucket. Dip a scrub brush into the mixture and scrub down the fireplace, removing all soot and other grime.
Rinse the fireplace well with clean water. Wipe it dry with old, clean towels. Allow it to air dry.
Tape off any areas you don't want to get finish on with painter's tape.
Dip a nylon brush into a can of high gloss polyurethane. Apply a thick coat of polyurethane to the fireplace, giving it 24 hours to dry. Buff it with 0000 steel wool before repeating. Repeat three times for maximum shine.