It’s been my experience that the AK product actually works better than the old Blacken-It. I have no idea why, but the AK product seems to create a better ‘browner’ color tone and does so faster than Blacken-It. Regarding the missing spots, that is completely common. I think that has been attributed to various inconsistencies in the pot metal used to make the tracks, spots where that tiny area of metal isn’t as susceptible to the action of the chemicals. It does not appear to be due to anything else, such as inadequate cleaning, insufficient time in the chemical bath, or air bubbles that many folks like to attribute those white spots to. I’ve never found them to be a problem, as I always do more weathering after the chemical treatment, and I’m careful to make sure that any mud/dust/pigments cover those spots. Alternately, I have painted those metal tracks when blackening solution wasn’t available, and that works fine, too!!
You can also use black patina from any stained glass supplier. I’ve used it quite a few times.
