Good advice from my friend, Tim Carter of www.askthebuilder.com
I discovered many years ago it’s a mistake to save the paint in the cans from the paint store. Not only do you have to deal with lots of air in the cans, but you also have an issue with rust at the lid lip. The excess air in a paint usually causes a nasty skin to form on top of the paint.
What I’ve discovered that works best for me, and you may have an even better idea, is to use the inexpensive food-storage containers you can buy in the grocery store. They come in different sizes and the trick is to fill one nearly to the top with the leftover paint.
If the paint is water-based, I always add an ounce or two of water at the top of the paint before sealing the lid. This prevents the skinning effect. Using a permanent marker, and I do this before, filling the container, I write on the side of the container the room the paint was used in, the mfr., paint type, paint name, formula if available, etc.
Yesterday I had to touch up many spots in my living room. I painted this room about ten years ago. If you were here with me today, I’d bet you $100 you couldn’t find one spot where I touched up the walls. The paint matched perfectly and it’s consistency was just like the day I opened the original can. Suffice it to say I store this leftover paint indoors at about 65 F.