I prefer using Spottone for retouching, but unfortunately it is no longer made. The secret it to use a #000 brush (triple zero size). Do not use it like a paint brush, but instead use it to slowly dab or place dots of retouching fluid in the offending spot. This is way the action is called "spotting." You should slowly build up the density of the spot to match the surrounding area of the print instead of trying to match the tone all in one spot. Presently I use Marshall's neutral black. Henry Horenstein's book on basic B&W photography has a good chapter that addresses spotting.