I made a table about 20 years ago, the top is four pieces of hardwood I picked from a site rubbish pile. There were always gaps between the boards (from about 1 to 10mm) and I've never had complete success filling them and sealing rhe table. Its always been a very utilitarian table, cutting up food etc on it, so I probably can't seal the whole table top, but I would like to be able to fill the gaps between the planks in a more permanent way. Thanks very much.