Buy colorless damar crystals in a clear bottle--any color in them will color your varnish. Small specks in the crystals can be decanted out after sinking to the bottom, if necessary.
Buy only real turpentine--no substitutes--from a hardware store. Make sure the container says it is pure gum spirits of turpentine and that it smells sweet and natural. It should be perfectly clear.
Place 5 lbs. of damar crystals into the cheesecloth. Wrap them in the cheesecloth and tie it with the string. Dunk the crystals in 1 gallon of turpentine.
After the crystals have dissolved, slowly pour the mixture into the new jars. Be careful to leave any solid material that may collect in the bottom of the original jar from the cheesecloth, string or that was not trapped by the cloth.
Dilute the varnish with an equal amount of turpentine for a final varnish. Use 2 parts turpentine to 1 part varnish for a retouch varnish.
Apply the varnish quickly with a soft brush in a thin layer. Too thick of a layer will be very sticky and hard to work with. Reapply another thin layer later if necessary.
Let the painting sit for a month as the varnish dries. If the black areas in your painting have gone flat, you may need to put another thin coat of varnish on the whole painting. Do not spot varnish.