From Typewriter Wiki

The platen is a cylindrical roller, usually made of rubber or cork, that serves as the printing surface of most machines.



Many typewriters, particularly those manufactured before the 1950s, will suffer from hard platens. For these, the only long-term solution is to replace the platen rubber with new rubber. Properly recovered platens will have a hardness around Shore A 86. When working on a typewriter, it is recommended to clean the platen with a rag and rubbing alcohol. That will help it have more grip to the paper. If you're using a machine with a hard platen, inserting backing sheets is strongly recommended. They keep the firm rubber from sustaining damage or chipping as a result of the type slugs hitting it.

Temporary solutions exist in the form of rubber softeners, but these tend to be short-lived and damage the platen over time.

JJ Short and Associates of New York recovers platens.