I don't have experience with "Rubber Renew" but an ebay seller occasionally sells a product called "Drive Wheel Cleaner" for the rubber rollers in tape recorders, and I have some of that. I've used it on tape deck capstan rollers, turntable idler rollers, discolored rubber grip surfaces on vintage camera lenses, and on a couple of turntable rubber platter mats. It actually was the least effective on the platter mats. On one of the most stubborn platter mats, from an old (not a classic early) Garrard, I finally resorted to using a cheap off-brand automotive tire shine spray. Not knowing what was in that stuff, I cleaned any excess product off of the mat after it appeared to have completely dried, then I let it dry further for 24 hours before I dared put a record on the mat. Have you tried scrubbing the mat with a soft toothbrush and a gentle soap solution? That's probably do with a mat for a good machine.