https://3dtypewriterparts.com/

Polt, Richard. The Typewriter Revolution: A Typist’s Companion for the 21st Century. 1st ed. Woodstock, VT: Countryman Press, 2015.

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The white gunky substances seen on the plastic keys of old, unmaintained typewriters generally isn't mold, but plasticizers coming out of the plastic. These can usually be cleaned off using simple household cleaning products or if necessary heavier cleaners (Goo Gone, alcohol, naptha, etc. - test these on hidden parts first to ensure they don't react with or destroy the plastic or remove the paint of the key letters) followed up by light waxes or polishes and buffing.

Wiping dirty plastic typewriter keys with a soft cloth and then waxing/buffing them can bring them back to life.

Cleans them but probably doesn't repair them.

https://www.classictypewriter.com/typewriter-shop

reply regarding painting typewriters at https://www.reddit.com/r/typewriters/comments/1cflyf2/help_identifying/

It's been a while since I've done it, but I've sent vintage metal filing cabinets out to have them powder coated with stunning results. If you have someone local who does this, they'll be able to handle the details and give you color options. It may be best to give them the individual parts of the frame you want done and not the whole machine (especially so they don't lose or destroy anything vital). You can get some interesting colors and still have the older vintage look.

I've also been contemplating doing a brushed steel finish and several layers of shiny clear coat. I've done it to a few desks before (here's an example of a table with a brushed/enameled top though it's got a slab of glass on top too), but haven't done it with a typewriter (yet).

Depending on your area, you might find an auto repair artist who could strip the case down (sand blast/bead blast) and give you some real artwork including ombre paint, sparkle, racing stripes, etc. Just for fun, how cool would it be to have a matching "Jerry Orbach typewriter" if you had a Jerry Orbach car? If you're a Star Wars fan, it could be cool to have a typewriter done to look like either R2-D2 or C-3PO, for example. Or maybe cover a 1977 Smith-Corona Galaxie 12 with brown faux-Wookie fur and a bandolier Chewbacca-style!!! If you're going in, you may as well go all-in, right? (But definitely stop before you end up restoring one of the old black batwing-style Oliver's to fit it with a Darth Vader helmet dust cover...)

If you go with straight paint, your local paint shop can recommend the best combination of primer and paint formulation for painting onto metal. (The small sample pint sizes of paint may be more than enough to do a single typewriter.) They'll give you more color options than you could possibly want. You'll want a high quality paint brush and some paint thinner so that when you apply, the finish comes out buttery smooth as it dries. Various spray paints may be options as well, though here you may not have as many color options.