Jar lids tend to rust when left in a damp, humid atmosphere. Most modern containers are treated to resist rust, but this is not the case for many older jars. The problem is exacerbated in the case of a metal jar and lid, as rust will form on both the lid and the threading. Fortunately, you can open virtually any rusted jar by removing rust particles, lubricating the threading and using a rubber jar opener to get a firm grasp on the lid.
Run the jar and lid under a stream of warm water while scrubbing the edge of the rusty lid with a small wire brush. Pay special attention to the point where the lid meets the threading on the jar.
Rinse the jar thoroughly and dry with paper towels.
Turn the jar upside down and spray a small amount of WD-40 at the edge of the lid. Wait a few minutes to allow the lubricant to run down into the threading.
Place a rubber husband over the jar lid and twist in a counter-clockwise direction.
Wipe down the threading and rinse the jar lid to remove excess lubricant.