Yanni Giftakis recommends melting crayons into the letter groves of typewriter keytops to replace their missing paint with a heat gun, much the same way D&D players will fill in the groves on their dice.

This is similar to Richard Polt's recommendation "To restore faded paint on keys and scales, try Lacquer-Stik Fill-In Paint."

via https://www.facebook.com/groups/typewritermaintenance/posts/3831371387100282/