So you just finished your latest DIY project, repainting the dining room. The work went well, but now what do you do with the extra paint? One option is to keep it around in case you'll need it again for touch-ups, but you might also just want to get rid of it... How do you proceed?
Contrary to what I've seen done by some people, you do not pour extra paint into a storm drain. Not only is this illegal, but it's a major health and safety risk for people, animals, and the environment. But you knew that.
Depending on where you live, your city may consider water-based latex paints and oil-based paints differently with regards to their disposal.
Water-Based Latex vs. Oil-Based
Water-based latex paint, besides being a friend of the DIY-er because of its faster drying time and elasticity, is also easy to clean up. As the name implies, most of the liquid in water-based latex paint is water. For this reason, some cities don't consider it a hazardous material. If this is the case in your city, then simply let the rest of the liquid evaporate and then throw the can in the trash. Alternatively, you can use a paint hardener, or mix in cat litter or sawdust to absorb the remaining liquid and then throw it away.
Oil-based paints are a little more tricky. They are always considered hazardous waste. This means you can't just throw them in the trash and send them to the dump. Again, not only is this illegal in most cases, but also puts your community at risk.
Contact Your City's Department of Public Works
Your city's department of public works will have information on how to properly dispose of hazardous materials. Most cities run household hazardous waste disposal programs. There may be occasional neighborhood pick-ups, collection events, or permanent centers for hazardous waste disposal. San Francisco even has an ingenious iPhone app called EcoFinder which gives you information on how to dispose of almost all types of waste and shows you collection sites nearby.
Other Good Options
Probably the easiest and most altruistic method of disposing of extra paint is to give it away for free on Craigslist, where there are always people looking for free stuff. Your city might also host a program for redistributing materials which you no longer want but which someone else might. For instance, Los Angeles County hosts the innovative Los Angeles County Materials Exchange. Residents of LA County can use the website to browse or post various materials.
Also, just as a general safety precaution, make sure if you do keep extra paint around, you don't store it in food containers (old soda or tin cans, for example). According to the LA Department of Public Works, “Children have died from drinking chemicals stored in soft drink and juice bottles.”