Sand the mantel by hand with 100-grit sandpaper. The entire surface should be rough to the touch when you're done. Vacuum up any sawdust.
Apply primer to the mantel with a paintbrush. Coat the whole mantel with the primer and allow it to dry for at least 24 hours. Rinse off the paintbrushes.
Cover the mantel with high-gloss paint. Use large brushes for the flat, wide areas of the mantel. Use smaller brushes for the designs or hard to reach nooks.
Allow the mantel to dry for at least two to three hours. Apply a second coat of paint over the first coat. A third coat can be applied if you wish.