WD-40 sprayed all around under the rim, Navel Jelly, or a similar silicone spray/rust remover will loosen them up enough to remove the rings. You may have to let it set for a bit to work its way in and the lid will probably come off with it too so do it over the sink.

Dave

I know this is 10 months after the fact, but I found that an oil filter wrench works well on old wide mouth rings. I'd never use it on newer rings that might be reusable because it would dent the ring. I just bop the jar upside down on a wood surface like the bed of my flat bed trailer where I was sorting them, and then it loosens up quite a bit. Most of the time I don't need the wrench, but there were a few that I did.

I'm also a little late but here is how I do it.
If the lids are badly rusted on, no need to lose any of them in spite of this.
Set jar on firm base.
Use a small pair of wire cutters and make a small nip in the ring on the top of the jar.
Use a small pair of needle nose pliers and grab the little tip of metal lifted from the "nip" in the metal above, and start to roll the pliers as if you were opening a sardine can.
Keep rolling the metal to the edge of the band and roll completely through the rolled edge at the bottom.
If the metal tears at midway, just pick up a piece of the torn metal with your needle nosed pliers and start rolling again until you are through the rolled bottom edge. Just make sure you keep the pliers on the metal band and not the glass threads (they will chip).
Pounding and beating or using big pliers will/may remove bands but you will break some jars.
Using the method I have outlined, you should not lose any jars as you are not stressing the jars.
If you have rust on the jars or threads after band removal, use a rust remover designed for removal of rust in toilets. It usually contains a strong base or acid, so ware gloves and be careful of your eyes. This should just add a few minutes to the process (use an old toothbrush to do the application - moving very slowly so as not to splash).
Hope this helps.
Jim in So Calif
And yes, I have done this many times to jars left filled and out in the weather as long as 40 years plus.

Why not just take the rings off before you store? That's what I do.

Steve

why would you want to take a chance on a nasty cut for less than $10.00?
If you cant take them off easily, throw them away.

The reason you would want to take the bands off old jars is that many of them are collectable.
I got many Presto, Atlas, odd old brands, and old out of style Ball jars of so many different mold designs.
Some of us appreciate those old jars - I guess others do not.
You also cannot take off the bands when finished canning when you get jars from Craig's List from in my case, 40 or 50 years ago or more (about 325 jars for $10.00 and a pint of berry jam).
Jim in So Calif

Ditto Jim. I have my parents old canning jars, they are priceless!

I actually took my Dremel tool and carefully cut some of the rings apart!

yeah, I love Craig's List sometimes. Maybe too much. 3 or 4 summers ago now, I think I got well over 1000 jars from someone for about $140. P,Q,H. All. But they counted, boxed, and even left some in wooden crates and I hauled them all home at one time. So I gave them $160. Little did I know that the crates were loaded with black mold and never saw the inside of the house once I found out. I'll never use all the jars, but they are nice to gander at once in a while. I need a display cabinet to put some in. All total, I'm between 2500 and 3000 jars total now. Probably half antiques that won't ever see a canner again. I'll buy more if I can find them cheap enough! :-)

Used my bench grinder to very carefully in one spot grind the metal ring so thin that I could finish splitting it with a small flat blade screwdriver