https://new.reddit.com/r/typewriters/comments/1g3xtkp/platen_experiment_the_result/

Colloquial repair solutions for hard platens: <br /> - use backing sheet(s) - light sanding and cleaning with organic solvents - Brake Fluid soaking for an hour followed by drying time (see also: https://site.xavier.edu/polt/typewriters/tw-restoration.html) - Rubber Renue https://mgchemicals.com/products/electronics-maintenance/specialized-cleaners/rubber-renue/ - Methyl salicylate and alcohol mixture - Sanding and/or lathing and resurfacing with heat shrink tubing and/or by bicycle inner tube

Actual repair/restoration/recovery:<br /> - J. J. Short Associates, Inc. https://www.jjshort.com/typewriter-platen-repair.php

see colloquial advice at https://old.reddit.com/r/typewriters/comments/1ewb36f/rubber_renue_to_soften_platens/