I'm a newbie and I have been surfing some of the posts on this forum; don't know a lot about typewriter repair yet, but I have used Varn Super Rubber Rejuvenator for thirty years when I was in printing.  It is more accurately called a "super cleaner" because it will remove paper glaze, dried ink, white-out, grease, and dried oil.  It also removes some paint, dissolves plastic (as I found out to my dismay when using it on older letterpresses), and is quite expensive at $40-50 per gallon.  I would only use it on a platen off the machine, and then maybe once initially and then use less caustic stuff like alchohol to keep it clean.
What it doesn't do is "rejuvenate" the rubber.  It will deep clean, and give a very thin film of close to the original finish, but it cannot restore the rubber to its previous condition.  If it has flat spots, or grooves from type slugs hitting it over the years, they will remain.  The hardness will remain as well.  Platen rubber is too thin to do much with once it hardens;  Even press rollers, which were formulated to take far more punishment than platens, hardened and shrank.  We would grind the larger diameter rollers down 3-4mm to restore the original rubber, but they were around 20-25mm thick.  3-4mm is about the thickness of a portable typewriter platen.  As I've seen from a lot of posts here, recovering is the only solution for a super-hard platen.  Having said all this, I would like to know if anyone has actually used Varn on a platen;  I like to be very cautious when working on old machinery--and very cautious when using industrial strength solvents.