This only slows the drying, however. It doesn't stop it. So after several days I can no longer use those paints. They start to develop a dry film that gets chunks and bits in your painting. So I have to toss them out.
I haven't yet found a good method for cleaning dried paint out of plastic containers so I was just throwing the whole container out once or twice a week and getting new containers. This wasn't working all that well, though. Besides the fact that I can't eat enough turkey sandwiches to keep myself supplied with containers, the "green" side doesn't like the idea of throwing so many of these away. Because the plastic has been contaminated by paint I can no longer put the in the recycle bin. I don't like the idea of throwing so many plastic containers in the land fills so I wanted to think of a better, less wasteful, solution.
I know many artists line their palettes with paper so I decided to line my plastic container with paper. I didn't have any "proper" paper for such a thing so I looked around for what I had on hand. All I had on hand that I thought might work was parchment paper, normally used for baking. I cut a piece and put it in.
These worked out great! Now I can just throw out the liner and save the container for multiple uses. Much less wasteful this way. And, I don't have to eat so many turkey sandwiches! Not that I don't like turkey sandwiches but, really, with all that tryptophan, I'd be more likely to take a nap after lunch than get any painting done. :)
I don't just store my paints in these. I paint directly from them. That way I don't have to go through the hassle of transferring paint from palette to container each day. I realize some people may find my little lunch meat container too small to paint from, not giving them enough room for all their colors and mixing. But there are larger containers you can buy in the grocery store near the other food storage items like foil and plastic wrap. Stick with the cheap "disposable" containers instead of the more expensive Tupperware® and such. The cheap ones are just as good for this purpose and much less expensive.
Even if you need more space, you could still consider reusing your small lunch meat containers by using multiple containers for different color schemes. Put your sky colors in one, foliage colors in another, etc. Then you won't have your foliage colors drying out while you're only working on the sky.
NOTE: It's important to tape your liner to the container! I dropped mine a time or two and it could have been disastrous for the carpet had the liner fallen out and landed paint side down ... as Murphy's Law would have assured it did, just as a slice of bread always falls to the floor with the peanut butter side down!
Anyway, that's my silly little method for storing water mixable paints. If you have other ideas I'd love to hear them. Please leave suggestions and tips in the comments section below.