I just did my first opening doors a few weeks ago, and used sewing thread. I cut the doors out of my '40 Ford Sedan Delivery.
I like cutting with thread. It cuts very thin - no wider than the pane lines. It will "tend" to follow the panel lines but it can get off if you rush it. The thread broke frequently, but less so as I slowed down and reduced the pressure on the thread. It will still break now and then, so you just get more off the spool - thread is cheap.