Pour 1 gallon of water in a 5-gallon bucket, then add 12 oz. of TSP.
Clean the brick fireplace thoroughly with the mixture using a stiff-bristled brush until the grime and dirt is completely removed. Rinse with plain water and a sponge. Allow the bricks to air dry.
Rinse out the brush and 5-gallon bucket, then combine 6 parts of water with 1 part of brick and masonry cleaner. Mix together with a paint stirrer.
Scrub the brick fireplace with the water/masonry cleaner mixture to restore the shine. Allow the mixture to soak in for about 15 minutes, then rinse with plain water and a clean sponge.
Brush on polyurethane elastomeric sealer with a paintbrush to seal the bricks on the fireplace. Allow it to dry as long as recommended by the manufacturer, then use your fireplace as normal.