If your existing urethane finish needs extra protection but not a total refinishing, you can re-coat it with Minwax® Helmsman® Spar Urethane, provided you follow the three rules for re-coating: clean, dry and scuff. First, clean off any wax and dirt with a cloth dipped in mineral spirits.
You are correct that, with polyurethane (brushed or wiped), you can recoat without sanding -- as long as you recoat within about 24 hours. After the poly has cured (usually about 3-7 days, depending on the product), you then have to sand. This abrades the surface and allows a mechanical adhesion between the two coats.
If you want to change the sheen, you can re-coat with a different sheen over top. The second layer of top coat will replace the appearance of the first. Sand lightly with a used 220 sanding PAD to knock down the original finish before re-coating. It may take two or 3 coats to get complete coverage.
You can rub out all types of film forming finishes, including shellac, lacquer, oil based varnish and polyurethane, waterbased coatings and catalyzed or conversion coatings.
Before refinishing try rubbing it out with rotonsone or pumice with oil or water. You'll get different sheens with different combos of those. Even 0000 steel wool with a gel like Elbow Grease can give a nice low luster hand rubbed look.
After it cures you may use a paste wax for wood with a 0000 steel wool applicator. It will wax and dull the shine at the same time.
*How Do I Remove Brush Strokes from the 3rd Coat of High Performance Polyurethane? Sand down the final finish with a 220-grit foam sanding pad, and then add another layer of General Finishes High Performance PolyurethaneTopcoat. Apply liberally than you did previously without heavy back-brushing.
Prepare the polyurethane by reducing it about 10 percent with mineral spirits, unless you're using water-based polyurethane, which doesn't need thinning. Thinning the polyurethane makes it flow on more smoothly and reduces brush marks. Quick Tip!
Avoiding Sanding and Priming Prep
Surfaces with varnish, polyurethanes, or other sealants or finishes require sanding before any paint can be properly applied.
Waterborne polyurethanes usually require a minimum of 3-4 coats to be effective. Although there is some evidence that too many coats can result in a plastic-like finish, waterborne polyesters are thinner than other types and therefore require several more coats to provide the same protection for hardwood floors.
Urethane can be stored for a long time as long as its kept dry and sealed tightly. You will know that its been contaminated or moisture got in if, when you open it, it looks like cottage cheese.
Lightly sand the old polyurethane to create a rough surface for better adhesion, then clean thoroughly to remove dust and debris. Apply the new polyurethane in thin, even coats, allowing sufficient drying time between layers.
If I need to dull a finish, I typically apply ren wax with 0000 steel wool or very fine synthetic pads. A light buff with a microfiber cloth and done. Seems to work for me.
Make sure to sand the polyurethane between coats. Sanding removes drips, bubbles and other uneven spots in each coat. Dry the first coat for a full 24 hours. If the coat feels tacky or sticky after that, let dry until it feels completely dry.
You should use an automotive polishing compound and a buffer for big areas. Small areas will have to be polished by hand unless you have some funky buffing tools. Make sure the poly has cured for a minimum of 4-5 days at 70f. Do not try to polish an uncured film.
Liquid sandpaper, a.k.a. deglosser, is a chemical that can be used to rough up existing paint on walls and other surfaces by removing the gloss, making it possible to paint over what's left on the wall after use. This is good for hard to reach places or an object that you may feel would be awkward to sand.
Start sanding with coarse-grit sandpaper. It will strip away the top layers of polyurethane. Work methodically in the direction of the wood grain to avoid scratches.
Unlike regular sandpaper, it just removes gloss; it can't be used to smooth the surface, correct imperfections or as a paint primer. Liquid sandpaper is also referred to as deglosser and wet sanding. The advantage of using liquid sandpaper instead of regular sandpaper is that it's easier to apply and safer to use.
Urethane finishes are hard and require a scotchbrite pad or 1200-1600 grit wet sanding to dull them. Scotchbrite is my preferred abrasive for this as it is better able to cope with any texture and high or low spots just by the pressure you apply and achieve an even, satin finish.
Bubbles and streaks can ruin the smooth finish of your polyurethane floor. To avoid bubbles, make sure to stir the polyurethane gently and avoid shaking the can. When applying, use a high-quality brush or roller and apply thin, even coats.
Add a pea-sized amount of car paste wax to a polishing pad or cloth. Work in short, smooth motions until there are no visible scratches in the polyurethane.