Prime the wood with either a clear or white primer. Apply the specific stain or color of paint you desire. Minwax Sanding Sealer is a good sealer to apply after a stain or color, though it will be quite slick looking. Seal whichever sealer you use with a coat of Minwax Polyurethane to ensure the mantel is protected. Always double-check with the manufacturer, whether in the directions, on the can or by phone, to ensure that the product is safe to use on mantels.
If the brick mantel has been used for quite some time without a sealer, apply scouring powder with a stiff-bristle brush and scrub the mantel all over, getting into every crevice. Rinse the mantel with water and rags and allow it to dry before applying a sealer. Use either a coat of brick enhancer or paint to the porous mantel -- if need be, apply two coats, as it will absorb quite quickly. Allow it to dry according to the manufacturer's instructions. Paint two coats of brick sealer over the top.
Clean the stone with a commercial formula developed for cleaning the porous surfaces of stone. Remove all residue according to the cleaning manufacturer's instructions. Allow the mantel to fully dry. Purchase a stone sealer from your hardware store, checking the fine print to see if it is suitable for heated areas. You can purchase the sealer so it is not visible on the mantel, or adds a darker or lighter tone, or adds gloss when it dries. Tile Guard Natural Stone Premium Sealer is a good sealer for almost any stone, but you should use a more solid sealer for porous sandstone. Use stone-enhancing sealers for mantels that require more depth and color.
Evaluate what type of tile was used on the mantel. This will determine what kind of sealer used. You can find sealers for tile at almost any hardware store. It is best to phone the manufacturer of the sealant you are looking at before purchasing it to ensure that it is OK to use on mantels.