So yeah, I purchased a typewriter from a gentleman on Craigslist a few nights ago. It’s a Royal HHE, c. 1956, and it works quite well.
However, some of the typebars had so much gunk that the output looked just horrible:
Even cleaning even with the firmest bristle toothbrush you can buy at Walgreens doesn't help. One trick that has worked out quite well for me thus far is to clip the bristles on a toothbrush so that they're about 2–3 mm long. If you have an old cheapo electric toothbrush lying around (like a Crest SpinBrush), even better.
As you can see below, it’s resulted in quite the improvement:
There is still a little work to be done, particularly on the uppercase M and V. But what a difference a toothbrush can make!