When Jeff "the dice man" and I would talk about this I brought up the use of old perfume bottle sprayer to give it a light coat of sealer. Yes, using the nail brush would deloot the colored markings and streak them across the die. B A D. but the spray good. when I did pastels, I would use a can of lacquire/sealer, if you go to this can type watch out for the length from the die to the spray. On the pastel drawing if you got to close to much quickly was applied. BAD The out come you could already figure. But to far and you started to develope Hairs on the plan of the drawing. Practice if you plan to use the can spray, and don't get eager.
Second we thought was you had to keep checking the coated die to see if crackes had allowed air in, thin coats to seal.