to start with, you try as-much-as possible, to not have to "knock on doors" to begin with. Anyone knocking on the door will have the instant category of door-to-door salesman, or door-to-door missionary of-various-persuasions, etc.... And even though YOU are not "selling" something, or trying to "convert" folk, yet.... you will simply be lumped into that mental category of persons who just interrupted their breakfast or TV show.
Instead: catch them when they're on their front porch (so-to-speak). Eg. someone out doing yard work, working in their garage, unloading the groceries, sipping ice-tea on their porch, etc...... Even if that means scouting block after block of homes in older neighborhoods for a single one like that, and even if it means that such a one isn't particularly the better looking one.
Because the moment you can do ONE like that, it tends to "open up doors" to adjacent ones. Introduce yourself as "their neighbor from a few blocks away...". Have your detector in hand (so that they have the full mental conception of your intention/request). Have a buffalo nickel and a few wheaties you "just found at the house/yard on the corner" so you "have a suspicion that other yards on this block, being equally as old, hold same potential". And you're "writing a story for a historical research you're doing", blah blah.
Make sure you casually get the name of the person whose yard you got onto. Then as you're doing this one, you see someone else arriving home next door or down the street. Thus you "drop the name " of the first one, showing what you just found there (the buffalo and IH you brought with you from the start), and were wondering "if I can try this yard as well".
And I always add: You're welcome to any/all coins I find, as this is just for research hobby I'm doing. I have NEVER (well, only once) had anyone actually ever exercise the option to obtain all the coins I'd found.
It also helps if you are plugged in to museums and historical societies in your area as docent worker/volunteer. Then you have a name badge (for your 6 hr. per month desk-watching duty) to "flash" as you describe the historical article you're writing on the area. And if you go this route/yarn, be sure to have a little knowledge ahead of time as to when their house was built, and something tantalizing about "the stage stop that used to be at the end of the street", etc....