If your mantel is stained wood, you need to remove the old finish so you can add new coats of stain. If your mantel is painted, there is no need to remove the existing finish completely if you’re just going to repaint it, but you do need to sand it so that the new paint finish will adhere better.
You can remove the existing finish with either sandpaper or a chemical stripper, or a combination of both. Use a medium-grit sandpaper or sanding sponge to sand off the old finish and smooth out any imperfections in the wood. Sand carefully around any decorative details and curved areas. Always sand with the grain of the wood. Any swirl marks caused by sanding will stand out when you stain. Repeat the process with a medium/low-grit or low-grit sandpaper. Use a tack cloth to remove sanding dust.
Using a chemical stripper is probably the easiest method for refinishing a mantel. Depending on the kind of stripper you use, apply it with a clean cloth or paintbrush, or spray it directly onto the wood. It will begin eating away at the old finish almost immediately. Use a paint scraper, sponge or a rag to wipe off the old stain. Once you've removed the old finish from the surface, use a clean rag to wipe down the surface so that the wood is completely bare. Let it dry.
Always follow manufacturer’s instructions and wear rubber gloves and protective goggles when using strong chemicals.
Never strip furniture near an open flame as stripping chemicals can ignite.
Open windows and doors to keep the room well-ventilated.
If you use a chemical stripper to remove the old finish, it’s a good idea to lightly sand the surface afterwards with fine-grit sandpaper. This will ensure a tighter bond between the wood and the new coats of stain. Then use water, mild detergent and a sponge to clean the entire surface of the mantel. Let it dry before continuing.