We use a product called Flitz to make plastic (and metal) key come back to shiny. Do not dremel the polish though. It will ruin your keys and you'll be looking for spare keys. Keep the solvents like acetone away from that SC Sterling , if that is the machine you're working on. The crinkle paint will get soft and either rub right off or you can be left with permanent fingerprints in your machine's paint. The best cleaner for the keys and the painted case are soap and water. Elbow grease and a decent toothbrush.
If you take the case of the machine off the chassis, make sure to replace the now-rotten or gooey rubber washers which sit between the case and the chassis at each point where a machine screw attaches. You can make these out of a bicycle inner tube (thin, not puncture resistant) and use a very small leather hole punch, about 3mm to get the size right. 
If you decide to use rubber rejuvenator on the platen you MUST take it out of the carriage or any overspray will ruin your paint and will make your keys dull and white, again due to solvents in the product.
When you get it all back together, after flushing the chassis with mineral spirits or naptha, use a small amount of heavy oil (I prefer 90wt hypoid gear oil) on the carriage bearing track, using a brush to get the oil as close to the bearings as you can, then they will spread it naturally. Don't over-oil the machine, better less than more. Use a good watchmaker's synthetic oil for every other metal-on metal interface but not in the typebar segment. You may have to use a few applications of oil softening solvent in there, like mineral spirits, along with an exacto knife blade to scrape the 60+ years of garbage out of there. A low pressure air compressor is a big help. As a camera repairman, I use a rocket blower on my typewriter bits as well and it works great. Good luck.
Phil Forrest

Agree with not using a power tool, like a Dremel. Even at its low speed I have had it melt keytop plastic. The polish I use and like is Simichrome, available at motorcycle shops (among other places, probably). It works as well on plastic as on aluminum.

Duane Jensen with Phoenix Typewriters has a YouTube video showing cleaning type-slugs and the segment.  He places shop towels over the key-tops, platen, and even under the type-slugs (between type-slugs and padded rest bar) to keep the chemicals from harming surfaces and components.  IIRC, he uses lacquer thinner which can be quite harsh.  I use denatured alcohol which is more gentle and less worrisome.