Use a metal putty knife that you can hold and when you push down at an angle, the blade will bend flat against the membrane, and move flat blade part just above the membrane (hope that makes sense).
You may leave an old thin layer of old sealant.
Try to get enough old stuff started so that you can grab and pull up as the blade is cutting thru underneath.
If the membrane is stretching as you are cutting, come in at the different angle or direction to prevent the membrane from stretching.
Use the acetone sparingly, to clean.
Pat
