No, you should never pour paint thinner down the drain. It’s a toxic chemical that can harm the environment and wildlife by contaminating groundwater. Due to its toxicity and flammable properties, it has to be disposed of in a careful and responsible manner.
Here’s how to dispose of dirty paint thinner correctly:
Take any rags or towels that are soaked in paint thinner and put them in a metal container with a tight lid. Rags that are soaked with paint thinner have the potential to combust in the air, which can cause a huge fire. Fill the container with water and take it to your local hazardous waste disposal site.
Instead of pouring your dirty paint thinner down the drain, leave it to sit and separate. You can soak tools and brushes beforehand. Be sure to label the jar of paint thinner clearly, so nobody consumes it. Over time, the paint and other contaminants will slowly sink and eventually settle at the bottom of your container. The length of time it takes for this separation to happen completely depends on how dirty the paint thinner is. It can take anywhere from a couple of days to several months.
Once the dirt has completely settled at the bottom of your container, pour the clean top layer through multiple coffee filters into a clean glass jar. Be sure to leave a small gap at the top of the new jar and seal it tightly. Once again, be sure to label the container clearly to avoid accidental consumption. Also, be sure to always wear heavy rubber or nitrile gloves when handling paint thinner.
Next, leave the container open and let it dry in a well-ventilated area. Add cat litter, sawdust, or sand to speed up drying. Keep this container out of the reach of pets and children, and away from heat, flame, and flammable materials.
Once the material is solid and completely dry, wrap the paint thinner in newspaper, then seal in a plastic bag. You can now throw it away in the household trash. It’s important to note that many waste collection services will accept this, but there's a chance yours requires you to bring this to a hazardous waste site. You can call yours to confirm this.
Next, throw away all empty paint thinner containers. If there is still liquid inside, or a large amount of dried residue, take the container to a hazardous waste collection site. Most hazardous waste facilities will accept paint thinner in a sealed metal or glass container. Contact your local facility if you have a large amount of waste to dispose of.
Alternatively, if the paint thinner is clean and you just have an excess amount, you can give it away to someone who needs it. Offer it to a friend or post an ad online. If this gains no traction, you could try donating it to a local organization that could use it for renovation projects. It’s best to try not to buy more paint thinner than you need in the future. If you run a business that uses a large number of hazardous materials, contract with a private collection agency to pick up your waste from the worksite.