Many recommendations from others on here and THT, but what I found worked best for me so far due to my rubrail was in pretty rough shape in regards to aging, the Interlux 355 worked the best hands down in removing the aged layer of rubber. I used some cotton towls, rubbed them with a very firm action and fairly wetted cloth and you could feel it removing the layer of aged rubber. It left a very smooth surface that looked new!
Then, after a wash/rinse and following week, I put on 2 apps of the Mop-n-glo and wow, it really looks nice.
Now, after a few months and several trips out in the salt with wash/rinse cycles, it still looks great!
I thought this rubber rubrail was shot and needed to be replaced, but the 355 seemed to melt off the aged rubber layer giving me the new layer to now protect.
I did try acetone, 315, mineral spirits, alcohol, and some other products before finding out about the 355. Some worked and other did not at all, but none of them did as well as the 355. I say this just in case yours or anyone's is in bad shape, give it a try! It is a little on the expensive side, but what isn't nowdays!