I'm getting new front brake pads.
(powerstop)
My rotors seem to be fine and in specs. The only thing is that rust build up when it rains
I hear it's perfered to buy new one.
If your rotors are in spec and dont even need turning, there is no reason to buy new ones.
When they've been turned once before by a shop its recommended sometimes a replacement though (because every turning make them a bit thinner) but from your description it doesn't sound like thats been done or is being suggested to be done..
Yes, bare cast iron will rust when it gets wet and sits. They do make treated rotors for vehicles that are in areas where corrosion is an issue, like states where they salt the roads during the winter. You can get them at Rockauto.com.