So here's the interior of the mirror box on my newly acquired M645 (with the mirror removed and put aside for safe-keeping): It's a bit of a mess. Bits of the grey goo covering sides and bottom (and the back of the mirror) are crumbling and going everywhere. Application of a fingernail turns it to mush, and use of lighter fluid makes it slimy muck. I've resealed a pile of cameras, but never encountered this stuff before - the mirror boxes have always been matte black painted, not coated in mush. I can get it off (I'm patient, and expect to spend the next few weeks doing so) using lighter fluid and a bamboo scraper, but I'm wondering what's best to replace it with. I bought a light seal kit a while back from Jon Goodman that included black felt (much of which has been used up on doors). That's probably the most appropriate material from the stuff I have, but I'm thinking it might be a tad thick. Otherwise I found some stuff that's made for lining telescope tubes: http://www.fpi-protostar.com/flock.htm That looks pretty cool, and appears to be thinner than Jon's felt. What are people's thoughts?