The chain on my road bike has about 1k miles on it. It formed a little rust after a couple rainy rides at the end of the season when I stopped riding.

I am wondering if I should try and clean the rust off, or just put on a new one? And if cleaning is the solution... what is the best way to remove the rust?

Also, compared to a new chain, there is only 1-2 mm stretch on the rusty chain. How much stretch warrants the need for it to be replaced?