Hi All! Working on our 1904 Millworker's Cottage up in the Pacific Northwest, and we also uncovered shiplap + tongue in groove wood plank walls and ceilings. We peeled off wallpaper last summer (was in very bad condition, otherwise we would have tried to salvage), then sanded (this took about 3 months). I filled cracks and nail holes with wood putty, then we painted the walls white and shellacked the ceilings.
One year later, and we are looking to wallpaper back over the walls. Why?? As mentioned above, the wood was never really meant to be left exposed. While we love seeing the teeth marks from the saws, our walls are very uneven in gaps and cracks, and it really just looks sloppy and unintentional. Also, our house is much more drafty. The wallpaper acted as a layer of insulation. Without that barrier, air moves freely through the boards. Our house was built extremely well, and we'd like to honor it by restoring original features, such as wallpaper. Exposed shiplap is trendy right now, but as stewards of our home, we hope to do what is in the house's best interest in the long run. Best of luck!