Sand down your new or used mantel with an electric hand sander and 220 grit sandpaper. This is important to do so that you smooth any rough spots in the new unfinished mantel. If you have an existing mantel, it is important to rough up the product that is on the wood so that the new finish will adhere to the surface. Wipe down the mantel with a tack cloth after you are done sanding to clean it of any dust particles.
Stain the mantel if it is unfinished. Choose a stain that will complement your paint color. This is important to do, so that when you antique the mantel the stain will show through once you have painted it and then sanded down the mantel. If you are staining an already used mantel and there is already a stain on it, you do not need to re-stain it, just rough it up using the method as mentioned in the above step so that the paint will adhere to it. Allow the stain to cure for 24 hours.
Rub paraffin wax all over the mantel. Focus on the edges and places where there would naturally be an aged look. Paraffin wax is used to help lift the paint back off of the mantel. Paraffin wax can be purchased at most grocery stores in the craft or candy section.
Paint the mantel. Allow the paint to dry completely before adding another coat. When antiquing it is not necessary to do anymore than two coats since you will be doing a lot of heaving sanding and a lot of the underneath wood will be showing anyway.
Sand down the mantel after the paint has completely dried. Focus the sander on the areas where there is paraffin wax. These areas should be obvious since there will be uneven lines in the location of the wax. When you sand over them, the sander will lift the wax back up and the under wood will be exposed. This will leave a very natural aged look of peeling paint. Wipe the mantel down with a tack cloth.
Wipe the polyurethane over the top of the mantel using a 3-inch foam brush. Allow the polyurethane to dry for 24 hours. Add another coat. Allow to dry for 24 hours.