I've read a lot about platens, especially articles that deal with the the options you have when restoring them. Fortunately, none of my typewriters have a platen that is so bad that it needs to be replaced or recovered, but I do have a few that are pretty hard. As you know, a hardened platen is not good news for the machine's other components. They're hell on ribbons and slugs, and will even make a mess of the paper you're typing on. 

There are a few techniques for softening up a rock hard platen. A cheap fix is just to load a few sheets of paper in to soften the slug's blow, but I find that to be a cumbersome exercise, especially if you use the machine often.

Other techniques include sanding it down to freshen its surface, or even using various chemicals to soften the rubber. It may come down to something like that, but I love purpose-made solutions that come in a can.

Has anyone tried using of the platen rejuvination sprays that are available? I'm thinking of ordering a few cans - the products claim to clean and rejuvinate platen surfaces - but I want to know if it actually works first. With twenty platens that use a cleaning it would be a worthwhile investment.