Everyone - thanks SO much for your thoughts on this one.
Paul, I do that as well, I found in Houston too, the humidity makes it soup in a week. So I routinely add thinner.
I think I have found the culprit. The leather, when a little wet, does not adhere as well, and I think is letting in dust from when I sand the edges, turning it a dark brown. I did as Paul said today (two thin coats with drying in between and I did not see this line today. the leather was also very dry. I like to work it while it is still wet from wet form as it is easier - but I need to let it dry, glue in the welt, and then rewet it and continue.
Chuck just wets the section he is working as he needs to - and I think that is a very good idea to do for me from now on.
TF