Applying a polyurethane coating to a surface gives you a solid, see-through layer that protects the surface from the wear and tear of normal use without detracting from the appearance of the surface. In addition to offering protection from minor damage, a polyurethane coating can also add gloss to a surface or, when desired, can create a matt appearance without the glossy appearance of most finishes. You can create this matt effect by combining satin polyurethane finish with a dead flat finish, building up the depth with the satin then dimming what gloss there is with the dead flat.
Clean the surface that you're applying the polyurethane to with a pH-neutral cleanser and a sponge. Wipe the surface clear of any dirt using a circular motion to avoid grinding any dirt or debris into the surface through the sponge's movements. Rinse the surface with clean water, then pat it with a piece of cloth until dry.
Brush on a thin layer of water-based satin polyurethane using a paintbrush. Use long, even strokes with the brush and apply the polyurethane in lines across the surface. Overlap the line edges slightly by not more than 1/2 inch to ensure complete coverage. If covering wood, work along with the grain of the wood as you apply the polyurethane. Allow the first layer to dry for two hours until it's no longer sticky to the touch.
Sand the polyurethane lightly with the fine-grit sandpaper to remove any brush strokes. Use a circular sanding stroke to avoid leaving lines in the finish. Wipe away the sanding residue with a clean dry cloth.
Apply two more layers of the satin polyurethane using the same process as the first to build up the protective depth of the covering.
Cover the satin polyurethane with a moderate layer of dead flat water-based polyurethane. The dead flat will remove any slight gloss created by the satin finish and create the matt finish desired. Use the same application process as that used with the satin layers, allowing it to dry until no longer tacky and then sanding it smooth. Allow the material to dry after sanding an additional 48 hours to allow the polyurethane to cure completely before using the surface.
Work within a well-ventilated room while wearing a respirator to avoid inhaling the polyurethane fumes.