To waterproof a dining table, sand the surface until it's smooth and remove all the old paint. Then seal it with a coat of outdoor wood paint and at least 3 coats of sealant, such as polyurethane, until it is as smooth as you want. First, choose a paint that is made for outdoor use and that is suitable for furniture. Then clean the furniture of any dirt and sand lightly to create a better bond for the finish.
Using the adhesive cloth, wipe off any remaining dust. Then, with gloves, paint every surface that would be exposed to the elements. Store the drying part inside for a few days or a few weeks for best results. Before taking any wooden dining table outside, you must first prepare and seal it.
If there's already a finish on the table, you'll need to sand it down so that the outer sealant adheres to the wood. Use an outdoor wood sealer with a clear finish. I like Roxil Wood Protection Cream. It can be applied with a brush or roller and requires only one coat.
This will protect and waterproof wooden furniture for years before it needs to be re-sealed.