When I originally needed to mask slides, I had great difficulty finding anything that was totally opaque when faced with a 250W projector bulb. After trying various types of adhesive tape, I even spent some serious money on a roll of 'proper' slide masking tape (about £20 for a small roll) and even that was no good. Eventually the solution that was suggested to me was to use aluminium baking foil. Flatten it straight off the roll, then fold it in two and crease it to give a sharp, straight edge to work with. Using a light box, position the strip of aluminium foil on the film and hold it in place with a couple of small pieces of cine editing tape (which is perfectly clear, not that that matters, but has adhesive that won't ooze when it gets hot). You may, of course, need to move the film relative to the slide mount to keep it central. It's all very fiddly, and any small discrepancy in squareness, etc. will show up ten times worse when projected, so practice on a gash slide first. When you put each piece of foil on with the tape, don't press it down hard until you're sure everything is OK. Be prepared for a lot of small, crumpled up pieces of foil that didn't go on correctly! It's hard work, but worth it in the end. Oh - and if you're dismantling existing mounted slides, make sure you have a new supply of mounts ready.
Steve
