Where metal and wet glass meet, rust happens. Glass jars with metal lids are particularly prone to the problem, especially when you store foods or liquids in the jars. Remove that rust once and for all with one of several different household substances, depending on where the rust resides. Even a raw potato serves as a rust remover.
A Soak in the Sink
Fill the sink with water and a squirt or two of liquid dish soap. Soak the rust-spotted jars in the sink for 20 or 30 minutes, and then wipe them down with a dishcloth or sponge. If the rust doesn't wipe away easily, reach for a nylon scrub pad to scour it away. Remove as much rust as you can. Then rinse the jars and allow them to dry before placing lids on them.
Slice a raw white potato in half, slicing across the potato rather than down its length. Sprinkle baking soda or coarse salt liberally over the cut side, and then rub the potato, baking soda or salt-side down, over the rust spots. This technique works well if the rust is around the perimeter of the jar, left behind by a metal lid or band. If the rusty spots are inside the jar where they're hard to reach with the potato, cut the potato into a smaller chunk such as a 1-inch cube, sprinkling it all over with baking soda or salt. Jab a fork into the potato, and insert the fork and potato inside the jar to scrub the rusty spots. Wash the jar out afterward. Both salt and baking soda serve as gentle abrasives that scrub away spots safely, without chemicals.
If the rust is on the outside of the jar or in a place you can reach easily, slice a lemon into wedges and sprinkle salt over one of the cut sides of the lemon. Rub the salted portion over the rust, and then allow it to sit for 30 minutes or so before washing. If the rust is inside the jar in an area that's difficult to reach, fill the jar with enough lemon juice to cover the rust spots and allow it to sit for 30 minutes. Use vinegar if you do not have lemon juice on hand. Dump the liquid out into a bowl to reuse, in case the rust needs a second treatment. Sprinkle the inside of the jar with coarse salt, and fill it partway with ice cubes, swirling the jar around so the ice and salt rub the rust away. Empty the jar and wash it out. Pour the lemon juice or vinegar back into the jar and allow it to sit for another hour if the rust remains, and try the ice and salt treatment again.
Not Just for Dentures
Fizzy tablets designed to clean dentures also remove rust stains from glass. Pour water in the jar -- enough to cover the rust spots -- then drop in a denture-cleaning tablet. Allow the jar to sit overnight; then rinse it out. The rust may be gone without any scrubbing. If you do see remnants of rust, wipe the jar down with a nylon scrub pad in water, and then wash the jar out thoroughly.