One problem though: it's hard to get the invisible Japanese knot finish with string. You're supposed to gently pull the final knot into one of the holes, but my string was too thick and, try as I might, I couldn't get it to go in.
I did my brick-red sketchbook first and that was the worst, because I used the string as it came, in 6-ply, which was way too thick. Then I remembered that, when I did the test one in the summer, I split the string. I did the other two with 3-ply, which is not as strong, but seems OK. The knot still refused to go right into the hole, but I managed to tug it in half way, so you don't really notice!