Last time I bought paint, the clerk gave me a can opener.
Today when putting painting supplies away, I noticed this tool is also a paint can closer.
I assumed the flat end was the opener (which it is) and the other end was a bottle opener, for whatever reason in the 21st century when hardly any bottle cap needs to be pried off. Turns out the bottle opener part is for pushing down the can lid, using the sticking-out part as a lever. It works ok – probably really well on an unused can of paint – but I was dealing with a 19-year-old can, so a rubber mallet was more effective to get a good seal.
How to keep paint usable for two decades? Seal the can as tightly as possible, probably with a rubber mallet and not the closing tool, then turn the can upside down and store in a cool, dry area. The paint seals the lid completely. It will thicken over time, and can get sort of weird, but just strain it through cheesecloth or old pantyhose and you might be able to paint over a patch in a wall, as I just did. I bought what I thought was pricey paint for the inside of our house in 2002, and it still looks fresh and the leftover paint pretty much still matches after all these years.
One last tip: when you first invert that sealed paint can, put it in a cardboard box for a few hours to test that the lid is firmly attached and not leaking. Paint spilled all over your shelf and/or floor is the mistake I made so you don’t have to!