I live in a log home. It's new for me and last year there was a separation between two logs where I could actually see light through. I had it repaired on the outside by a log expert but the inside remained unrepaired so a couple of YouTube vids and I was an expert in Chinking. First item to get was Backer rod to fill the gap.  I bought the 1/4 in but I think it was too small, so I went up to 1/2'.