Soak it in Must For Rust for a while and the rust will dissolve, I take small parts and drop them into a plastic conatiner and then pour in the MFRust until covered. Has worked great for me for a long time. I get it from home depot. It's liquid like water and I pour it back into a separate bottle for reuse.
I soak badly rusted parts for a few days taking the part out every so often to scrape the rust scales off so the chemical can etch under quicker, Parts come out great. Always keep a couple bottles on hand.