My wife and I moved into our current home 27 years ago when it was only 5 years old. It was a stone and cedar home with the interior done in TG knotty pine boards. After 25+ years like all wood the planks have started to shrink and leave gaps at the T/G joints. Some as wide as 1/4". We're getting the house ready to sell hopefully by the end of this year and move into a 55+ community because I can no longer take car of the house and all the land. Some of the T/G planking is getting painted (please don't scream here!). After 30+ years of wood everywhere it seems to be too dark for our taste. As much as I always said I would never paint the wood planks, I'm starting to like the results.
The problem has been the separation between the planks makes the planks stand out as not being such a smooth installation. My solution was initially to take down the wood and re-install it.... Not gonna happen at this age. But.. after experimenting with wood dowels, specifically 1/8 and 3/16 dowels, I'm happy with the results. The dowels take up just enough space to make the separation disappear.
I had some pictures to insert but get the message "the file XXXXX.jpg is not a valid file". So you'll have to be happy with my description I guess.