A knight is never addressed as plain "sir" - at the first introduction, he is addressed as "My Lord" or "Milord" and any subsequent address is "Sir, followed by his firs name (eg Sir Lancelot).
Oooh no,no,no! Far too simple...
A peer is referred to as My Lord at first meeting but a plain old Knight is always Sir Firstname.
So, were I say Baron Clareprop of Heathrow, etiquette would require you to greet me as My Lord and then after as Sir.
However, were I Sir Clareprop Dreamliner, then etiquette would require you to call me Sir Clareprop when you first meet me and thereafter as well.
You notice I say etiquette requires this because being a good all round sort, I wouldn't care what you called me so long as the old G&T was available on a regular basis.