Allow the paint to dry:
Where?
Find an area which is protected from children, pets and rain but has an adequate air flow. A lock screened porch, a high shelf in a well-vented garage or shed may be suitable.
How?
The simplest way is to remove the lid and allow the liquids to evaporate. Depending on the type and quantity of paint, the drying process may take from several days to several months. This works well for small quantities (an inch or two in the bottom of the can) but larger quantities of paint take longer to dry and may require a different method.
Speed up the drying process:
1. Partial cans an be solidified in the original containers by simply mixing equal amounts of an absorbent material, such as cat litter, sawdust, plaster of paris, soil, vermiculite, or charcoal and allow to dry.
2. Apply another coat of paint to the surface your bought the paint for.
3. Paint excess amounts of paint onto cardboard or newspaper to use it up.
4. When drying paint in the can, stir it occasionally to break the surface film, allowing evaporation to proceed.
5. Pour thin layers (about 1" of paint) into a cardboard box lined with plastic. Allow the paint to dry one layer at a time, until all the paint has hardened.
6. For paint that has separated and cannot be mixed, pour off the clear liquids on the top, leaving the semisolid pain sludge in the can to dry. The clear liquid can be poured into a cardboard box lined with plastic and mixed with an equal part of an absorbent material and allowed to dry.
7. DO NOT dump it on the ground or down storm drains where it will travel directly to surface and/or ground waters.
8. DO NOT pour it down the drain. While small amounts of latex paint can be safely washed down the drain to a septic system or wastewater treatment plant, this practice should be kept to a minimum. Limit this to brush cleaning and cleanup.
9. DO NOT throw liquid paint in the regular trash. It may be released from the can when trash is compacted and may seep out of a landfill, contaminating ground water.
How should I dispose of dried paint?
When thoroughly dry, the remaining hardened material can be discarded with your regular trash. In this form, the material is stable and less likely to seep through to the groundwater below.