I find it helpful to blow out opened film holders with some canned/compressed air before loading them, but grit wouldn't explain scratching only transparency film. Assuming you send off your film, perhaps they are getting scratched in the box in transport? Versus just bring home b&w to process yourself (less bouncing around). AFAIK, pretty much all LF films use a similar/the same base, so that wouldn't be it. I'm tempted to lean towards the lab/transport issue, if it is just a specific process that you see scratches with.