It can be useful to take some mineral spirits, naphtha, or paint thinner and a tooth brush (or, even better, a brass bristle brush) to your typeface every now and then to clean the ink, dirt, paper, bits of ribbon, dust, etc. out of it. Doing it after changing ribbon is always a good idea. If you're really hard pressed, nail polish remover (acetone) or rubbing alcohol and a cotton ball along with a small pin tip for the loops of letters like 'a', 'o', 'p', 'g', etc. can be used.

How well (or not) your typewriter works from a print perspective can also change with the type of paper you're using, what your ribbon is made out of (usually nylon, silk, or cotton), how much ink it's got in it and how old/worn it may be. A good backing sheet behind your typing paper can also be helpful.

reply to u/kirrachristine at https://www.reddit.com/r/typewriters/comments/1dtuksy/one_letter_weirdly_thicksmudged/ RE: one letter weirdly thick/smudged