Thanks Maris. I'm aware of the method, and I do use it when on the rare occasion & for whatever reason (flare or even fogging outside the frame) I can't get the natural black edge black enough.
But, I never crop, I always print full-frame & shoot with that in mind, with the natural black edge showing that, and an unfiled/unadulterated 35mm mask always encroaches a little into the frame in any case, even if only 1/2 a mm or however much...... so, I guess I'm gonna be filing til my arm hurts