We have tongue and groove boards (not panels), likely pine, in our home office area. It's a natural finish, just some sort of an oil based finish on the wood. The problem is that there are some gaps, presumably because the wood has dried and shrunk over the decades. The largest is about 1/4 to 1/3 of an inch wide. Since it's not a painted wall, what's the best thing to use to fill in those gaps and make the filler disappear?