Assuming that this is not "patina", but significant rust- get a can of Kroil. It is made by Kano Labs, if you cannot find it locally, get on the internet. remove grips. Soak entire pistol in a generous slather of Kroil, wrap in plastic, let set for a day.
This is the best penetrating oil I have ever seen. beats PB Blaster. After it has soaked, start with a COPPER pot scrubber, like Chore Boy- or fine BRONZE wool (Chore Boy easier to find). Do wear gloves with Kroil.