Nearly Empty Can
Safely dry up any left-behind latex by mixing in a hardener (try Krud Kutter Waste Paint Hardener, $3.29 for 3.5 oz.; Amazon) or cat litter, then dispose. Some recycling centers will take empty latex paint cans, but containers that held oil-based paint are considered hazmat, so you may need to take them to a collection center.
A Little Paint
Got enough to keep around for touch-ups? Ensure paint stays viable by storing it in a sealable container that’s just big enough to fill all the way with your excess. A small metal can (like Behr’s 1-qt. Metal Paint Bucket and Lid, $3.42; Home Depot) is ideal; plastic containers may increase off-gassing. Make sure it’s clearly labeled—paint brand, color, plus where and when you used it— and store in a cool, dry place.
Almost a Whole Can
If there’s enough to justify keeping the can, be sure to retain the original label, too (note what you painted and the date). Use a rag to wipe any excess paint from the rim, then tap the lid closed with a rubber mallet.
Full, Unopened Can
Ordered more than you needed? Paint’s often nonreturnable, so consider donating. Contact the theater department at a local school or college to see if they’d like it for sets, or try PaintCare, which recycles unused paint.