Once opened how long should the rust seal last. I bought a tin used it 0nce went back a few weeks later and it is now rock hard. Is this supposed to happen?

Here is what we state in our directions: "DO NOT PAINT OUT OF CAN! It is recommended to use quality brushes but at disposable prices as cleaning them afterwards is difficult. After a good stir, dispense a working amount of RustSeal into a separate container (steel or glass is best) and seal original can immediately. Unused paint should NEVER be put back into the can as it will shorten its shelf life and cause pressure build-up possibly popping the lid. Keep can groove free of paint by using a scoop or KBS Paint Spout. Use heavy plastic wrap between the lid and the top of the can to stop metal to metal contact as RustSeal can permanently seal the lid. If there is any trouble resealing the can lid tightly, transfer and store RustSeal in another proportionally sized can or glass jar. Always store paint in a cool dry place. RustSeal may be refrigerated for storage but you must allow paint to reach room temperature before re-use."

If you do as suggested above, we have consistently found that RustSeal will last at least 6 months after opened.