Sort through your leftover paint. There are two general categories of paint: water-based latex and oil-based/Alkyd paint. Water-based latex paints are not considered hazardous household wastes and can usually be dried-up or solidified, and where local governments allow, it can be placed out with the trash. Oil-based or Alkyd paints, as well as paint thinners and other paint solvents are considered hazardous household waste and cannot be poured down drains or solidified and placed out with the trash.
After you’ve sorted through your leftover paint, begin opening each can and examine the contents. Both latex and oil-based paints generally have a 10- to 15-year shelf life, depending on how well it was sealed, on temperature fluctuations in the storage area and other factors. Paint that has been unopened has a greater chance of still being usable even after 10 years than a can of paint that has been opened in the past.
Don’t use a screwdriver to open paint cans. This can bend the lid and make it harder to close. Use a paint can opener.
Don’t worry if a “skin” has formed on the surface of the paint when you first open it — this is normal. Stir the contents of each can with a paint stirrer and brush the paint onto some newspaper or other disposable material. If the paint has clumps or has an overly thick texture, it is not usable. Organize again, by separating the usable paint from the non-usable paint. Set the non-usable paint aside for later disposal.