I'm by no means an expert in the history of either subject matter, but from my obversations I've noticed an influx of milk-spots starting in the 1980's, and only getting worse from there. While at the same time, I've also noticed an influx of more-frequent higher grades (a lot more 69+) of coins starting in that same decade. That's not to say you can't find a stray milk-spotted coin from the 1970's, or a great grade of one minted prior to 1980 -- just that there's a pretty noticeable upswing of both, beginning in the '80s. I don't believe it's a coincidence. And to answer your other question, no, not all old silver coins have a patina. I've seen plenty of obviously "dipped" ones, perhaps from using a product like this.. And these older dipped coins do not have milk-spots (nor does the product remove them).
Interesting video around a ranking on coins and spotting. One guess which was the worst? Lol [youtube]http://www.youtube.com/watch?v=SOzcyJ0ncf8[/youtube]
Interested to know, if you melted ten Canadian maples, all heavily milk spotted, into a sold bar, would the bar "milk spot up" ? Also, if you took a one once Canadian maple leaf round, covered in milk spot, and heated to 1,750 f, would the milk spots be gone, ....... forever ?