Left my p220 in its case inside my car for 2 days. Now there is a bit of surface rust. How can i remove this without having it refinished?

Soak the slide in Kano Kroil (a brand of penetrating oil, ACE Hardware carries it) and buff it with bronze wool.

Will that effect the finish in any way?

With that little rust, and it seems to be in little patches, I would spray it down with some WD40, wait about 15 minutes and wipe off with a towel that will remove some of the spots with little work. Then the remaining spots, reapply WD40 wait about 15 minutes and take a dental pick and gently rub the tip against the spots, thats how I remove rust on old C&R guns I purchase and it works well.

I would also take some oil or WD and clean the entire gun as you probably have some water still inside of it even if you can't see it, that should displace the water and prevent further rust. I'm surprised 2 days did that, maybe you should put a desiccator (like you find in jerky bags) inside of your case.

Soak it in some Break Free CLP.

Wait a little while.

Drink a beer and chill out.

Wipe off the CLP.

Most, if not all, that surface rust will come off. It'll still go bang, it's a modern black gun--it wasn't winning beauty pageants before anyway.

I will try all of those methods. It rusted soo fast because there is a leak in one of the seals on my car. Plus i live in Texas. To make matters worse, i had a trade lined up before this.

Aren't SIG-Sauers without stainless slides sort of known for rusting easily? I'm asking; I've read that several times.

My old agency issued them and used the older German guns in large quantities. Robar NP3 was a very popular modification for just this reason.