When gluing any backing with TB3, either wood or rawhide, I sandwich the backing material between the belly wood and another piece of wood. I put wax paper between the clamping board and the backing, so it doesn't bond to the backing. I usually use a clamping board of about 1" thick, and the same width as the belly wood. I also use rubber inner tubes, as they provide much more even pressure, and they're cheap! Before I strated sandwiching the backing material, I found my glue lines were poor, as the pressure on the backing strip was not even. I think this may be your problem. TB3 says to leave for 30 minutes, but I don't touch my glue ups, or unwrap them for 24hrs.