the problem with cold blues is it is only as good as the quality of the steel , for instance i have had miraculous results breathing new life in tent peg restorations (what i call rifles rusted to the point you have nothing to lose) , but some stamped parts not so much joy , the steel used on barrels and milled receivers is a high grade steel and with proper prep you can achieve remarkable results with cold blues , small parts such as your trap door its a crap shoot , to me though the best cold blue out there is Brownells Oxpho Blue