I seem to remember doing something similar to a 401 platter back in the eighties. Oops showing my age !! But was with a liquid plastic compound, obviously just on the underside of the platter. I think I painted on three very thin layers. I don't suppose this stuff exists now. Appeared to deaden the response when struck with a screwdriver. Swapped it for a 60hz platter, before I moved to Canada. The 60hz platter just feels lighter than the 50 hz version. I'm open to suggestions as to how to proceed further with the Garrard platter, but that's probably for another thread. One could also use dense foam injection, but this would be irreversible. HMMM, not such a good idea then.