De-glossing a clear polyurethane finish is a prerequisite to painting it or applying a new clear finish over it; if you neglect this important step, the new paint or finish may lift or bead. It takes a combination of cleaning, sanding and de-glossing chemicals to do a thorough job. On the other hand, you may want to tone down a glossy finish by cutting back the sheen. That's a different procedure that calls for steel wool and wax.
De-Glossing as Preparation for Refinishing
Step 1 Clean the Surface
Remove all grime, wax, and oils by washing the finished surface with soap and warm water. For best results, use a strong detergent, such as trisodium phosphate, or a substitute. Mix a solution of 1/2 cup TSP per gallon of water and wash with a sponge.
Step 2 Scuff the Finish
Wait for the finish to dry completely; then sand by hand, using 100- or 120-grit sandpaper. The goal is to leave visible scratches that enable the paint or finish you're about to apply to bond better. Since you're sanding the finish -- and not the wood -- you can sand with the grain, against it or in a circular motion, as needed. Don't apply enough pressure to sand through the finish.
Step 3 Use a Chemical De-Glosser
Apply a chemical de-glosser, following the directions on the container. The instructions usually direct you to saturate a rag with the product and rub it onto the surface until you feel the rag dragging. For best results, wait until you're ready to apply the new finish before using the de-glosser. The tackiness lasts for only about 30 minutes.
Toning Down a Glossy Finish
Step 1 Clean the Finish
Use mild soap -- such as dish soap -- and water. The goal is to remove dirt without etching the finish. An ounce of dish soap in a gallon of warm water is a safe solution. Wash with a sponge.
Step 2 Rub Out the Finish
Load a pad of 0000 steel wool with a small amount of paste wax and rub the finish. Rub the ends of the piece of wood -- going with the grain -- then rub the center in a circular motion. Turn the pad over or inside-out after you've applied wax to the whole surface and rub again, this time going with the grain over the whole surface. Lubricate with more wax as needed.
Step 3 Remove the Wax
Rub the wood aggressively with a soft cloth to remove excess wax. Change rags as the previous one becomes clogged with wax and keep rubbing until the cloth remains clean. You may need to sprinkle water on the surface to facilitate the wax removal.