Most ping pong players are more attached to their table tennis racket than other pieces of equipment, and rightfully so. A racket that is comfortable and familiar is a confidence-builder when playing against formidable opponents. Ask any player about buying their first serious racket, and they will remember it clearly. Therefore, it's natural to do anything possible to make a paddle last and remain in peak condition. Applying a sealer to the blade's surface is increasingly popular as a way to protect the surface when changing rubbers – pulling off the old ones can splinter an unprotected surface over time.
Because rubbers last about a year maximum in most players' opinion, it is a routine thing to change them. No matter how careful you are when pulling one-off, the blade surface comes under stress. Newer adhesives promise gentler removal, but even with them, you're pulling hard on a wood surface that can splinter. A clear polyurethane sealer, applied as a protectant, is an excellent idea. There is some odor and inconvenience during the application, but it is worth it in the long run. Anyone can do an excellent job with some patience and carefully following the product directions. So, there is every reason to seal a blade.
If you're going to be sealing a table tennis blade that isn't brand new, you'll need to lightly sand the wood surface after you pull off the old rubber; use fine-grit sandpaper. The sealer adheres best to a clean, smooth surface. If your blade is new, sand the surface lightly to rough it up just a bit. The sealer penetrates and bonds to the wood better when it's roughed up slightly. Before you open the can of polyurethane, make sure to read the application and use directions carefully. Notice also that the wood needs to be completely dry, and you need to use the product in a well-ventilated area for your safety.
If you're using polyurethane for the first time, the pros remind you to avoid the racket grip because sealing it can make it slippery. It's crucial to be aware of applying the polyurethane evenly across the entire blade surface, including the edges. The splintering you're trying to prevent often begins at the edges. Sealing them thoroughly will avoid future problems. When you're finished, and everything has dried, it's time to get out the fine-grit sandpaper again. You'll want to slightly rough up the blade surface before you apply the adhesive. Just like polyurethane, it bonds tighter to a somewhat rough surface.