Steve,
From what I can see of the coin, most of what is left seems to be
patina and not dirt. So in short I am not sure there is much more to clean.
However, try getting a something like a scalpel and GENTLY make small scraping movements on one
part of the coin, and preferably NOT one of the devices or letters. Yes it takes time if done carefully. Then look at the edge of the blade. Have you got any black/brown showing on the blade? If so and the coin is not looking damaged then maybe there is more that can come off, so continue. It is a slow process, and TBH, not always apparent where you should stop sometimes, but on coins like this where the details appears to be in the
patina more than any residual metal, it is the best way to do it.
Once you are sure that you are on the
patina, then stop and move to another
part of the coin. Again, that is easier said than done sometimes, but unfortunately until you have done it a few times you may go through
patina before realising. With a lttle practice and experience you will start to almost feel the difference through the scraping, and you will learn how to angle the blade to scrape with less chance of ruining the
patina. In essence I always view it as trying to "shave" the coin, and take a small layer off at a time.
Hope this rambling answer helps,
regards
Mark