I'd like to put etchings on my Bronica GS-1 focusing screen to mark 35mm panoramas. Is there a recommended way for etching the screens without damaging them?
Can you put down an overlay over it instead? For example, a transparency with the markings on it? That would avoid doing something permanent, let you swap them, etc.
'Etching' is damaging, if by damage you mean removing or 'relocating' material. To do actual etching, an X-Acto type knife would work. Depending on how wide a line you want, you can make a scoring tool from a blade, putting a little hook into it and then grinding the hook's width to the width of score you want.
Pencil, china marker. The overlay is a nice approach.
I have a couple of stock ones for my ETR series cameras-one plain and one with split-image. My favorite though is a plain matte screen which has been marked with black felt tip marker. The lines are more prominent than the etched lines of the stock ones and easily removed if I should decide to. I have not blacked out or cross-hatched the non-image parts of the screen; it just hasn't been necessary.
Marking is simple: using a small, flexible metal ruler, measure to find the horizontal centerline of the screen, then make small marks 12mm above and below center on either side, on the under side of the screen. Then using the ruler as a straightedge, connect the marks across the screen.
Quick, easy, cheap and effective.
You could print different ones such as a grid on transparent media with your computer printer. Cut them out and lay them as mentioned on the outer surface of the view finder.