I have a 100 year old porch floor, under a roof, but rain does blow on to the floor. Over the years, the tongue and groove boards have shrunk, leaving gaps of 1/16 to 3/16 of an inch between the surface of the boards. There is a living space underneath the floor, so it needs to be rain- tight. There is no tarpaper or other barrier underneath the boards. I am seeking suggestions on how to seal those gaps, prior to priming and repainting.