I’m new to the group.
I have prepped bryozoans and other specimens with really tiny spaces. Acid kinda works but it also affects the actual fossil a bit which I figured was counter productive. The best ways I found were either with a fine tipped dremel point (that too could damage the surrounding piece) or the old fashioned way if the material is soft enough…a safety pin. Plan to work on it over a longer period of time because your hand/fingers will get tired.
Every now and then a quick dunk in light acid solution (10% muriatic acid?) to see where things stand.