Rusty canning jars are a common occurrence; as metal rings get old they rust, leaving rust stains on the glass jars. When this happens the metal rings must be discarded; however, the glass jars can be saved. Rusty metal rings pose a danger to canned products as they are warped and likely will not screw onto the jar properly, thus not creating enough pressure to keep the lid of the jar down. Luckily the glass is salvageable.
Scrape as many of the large pieces of rust off with a sharp knife as you can. Scrape gently and at an angle; light force will not damage your jar.
Scrub the remaining rust spots with steel wool, also known as wire wool. Rub with medium force in circles, back and forth and around and in the rim to get off all the rust. Do not add water or a product while doing this, simply dry rub to release the stains.
Rinse the loosened rust off in soapy water. If not all of it came off, move on to an abrasive rust remover or use acid to help loosen the stain.
Select an acid to use. Virtually any type of acid can be used on glass; nothing you can obtain will melt it aside from heat. Acidic options include white vinegar, citric acid, oxalic acid, phosphoric acid, hydrochloric acid and sulphuric acid. Use any commercial cleaner that contains any one of these acids (or simply plain vinegar). Browse the ingredients lists on cleaners at your local supermarket to find an acid-containing product. Most commercial products labelled as "Rust Remover" contain oxalic acid. If you purchase pure oxalic acid, dilute it with water at a 10:1 water-to-acid ratio before using.
Dilute the acid (if using a pure acid) or use the compound cleaner, vinegar or lemon juice as is. Fill a shallow bucket or bowl with the liquid cleaner and place the jar topside down in it. For cleaners that are in powder form, mix with a small amount of water to form a paste and cake over the rust area.
Let the jar soak in the vinegar, lemon juice or household cleaner overnight (or for the whole day) but leave the paste on for only several minutes as it has a higher acid concentration.
Rinse the jar with water and soap and use a sponge if necessary to remove any remaining rust. Scrape off the paste with a paper towel and then clean and rinse. All of the acid must be removed before using the jar, so repeat the cleaning process.