All wood products will benefit from a coating of polyurethane, particularly if they are to be engaged in a high-use application such as a kitchen table. Polyurethane adds a toughness and a waterproof coating that are second to none, and the basics during polyurethane application will work with virtually any project that it covers.
The top of your kitchen table needs to be smooth and scratch-free for the best results. Sanding will smooth out any table top, and lower grits of paper, like 60 and 80 grit, will be used to remove deep blemishes while higher grits, 200 to 400 grit, will be used more as a polishing step than an actual sanding step. Always sand with the grain for best results. If you are re-coating a polyurethane finished table, use 200 grit sandpaper to rough up the surface. When applied, the polyurethane will fill in those tiny scratches and make your table look beautiful. After sanding, the table needs to be wiped off with a damp rag and then allowed to completely dry. Once dry, wipe off with a dry rag or towel to remove any remaining dust.
Decide which polyurethane type is best for you. Oil-based polyurethane is the choice of professionals because it is easier to spread, can be diluted for a sealing coat and adds a touch of beautiful golden color to the finished project. Water-based polyurethane is clear and easier to clean up but may raise the wood grain and not adhere correctly to stained wood. It is safe to say that for ease of application, go with a water-based polyurethane, but for the best looking results, go with an oil-based type.
Never shake a can of polyurethane because bubbles will form and they will be transferred onto your work. Always use a stir stick and gently stir the mixture thoroughly before application. When applying an oil-based polyurethane, dilute the first coat with one part paint thinner to two parts poly. Brush this on with the grain and allow to dry overnight. A water-based poly can be brushed on straight on its first coat. Lightly sand every coat with 320 grit sandpaper when it is fully dry. Not only will you smooth out any imperfections by doing this, but you will slightly score the surface of the poly and allow the next coat to flow into the scratches and grab on, making for a seamless finish. Continue from here, adding as many coats as you like, with four coats being about average. Always sand between coats and if any major bubbles form, pop them with a pin, then gently sand them out when that coat has dried. The last coat of polyurethane that is applied to your table top should not need any polishing. It will be smooth and complete. However, if you desire the shiniest and most highly polished top, or if there is a need to remove slight blemishes in the finish, apply an automotive rubbing compound exactly as you would on a car, using a circular motion. Let that dry, then buff it out.