While we don't recommend specific products, there are many chemical paint, varnish and stain removal products on the market. Some must be handled with great care because of the fumes that they generate, or their caustic nature. Wear protective gloves and safety goggles.When working indoors, be sure to have adequate ventilation. Take plenty of fresh air breaks. If you can smell the product, you are inhaling the solvents and should wear an approved respirator with an organic solvent cartridge. On the other hand, some products are far less toxic and are water-soluble. Some arenon-flammable and almost odorless. Because you are preparing wood to paint, rather than refinishing, good old-fashioned sanding may be all that is necessary. This is largely a function of how smooth the surface is now. Many older varnish finishes crack and "alligator". These are tough to sand.