By the way, I drive Nissan :-)(posted 8179 days ago)I think the streaking problem is related to halides released from film. When AgX is reduled, Ag(0) and HX form, but HX can further dissociate to get outside of the reaction, especially if the pH is high. If carbonate is not preferred because of gas formation, one can try Na2HPO4 buffer. (mix about equal molar quantity of NaOH to target at about pH 12.)