Buckets of colorful leftover paint.

It’s always a good idea to save some leftover paint. But if you don't store it properly, years later, when hallway walls, exterior doors, or window casings need touching up, you might find that it has deteriorated to a point that you can't use it.

Consumer Reports tests water-based latex paints because they're easy to apply and easy to clean compared with oil-based paints. They're also the most popular type. Leftover latex paint can last years, even a decade—or go bad in mere months. “When the paint can is bulging or the lid is puffed up, you know it's bad,” says Rico de Paz, who oversees Consumer Reports’ paint tests. “It means microorganisms are eating the paint and giving off gas, which generates pressure and inflates the can.”

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How We Test Interior Paints
Using nontinted base paints (paint before color is added), we evaluate how well each covers dark colors and resists stains. The results show whether the paint provides even and thorough coverage and can stand up to wear and tear.

For the coverage test, we apply one consistent coat to cardstock that has been painted black, and wait for it to dry. Then we use an instrument called a colorimeter to measure how well the paint covers the black paint. We do this twice per paint sample and average the results.

For stain resistance, our testers apply paint to a plastic panel. After it dries, they apply two lines of soils—coffee and a sootlike substance—and allow them to dry. Then they wash the painted panels, and use the colorimeter to measure the color of the paint on the rinsed areas. The closer the cleaned portions are to the original coat, the more stain-resistant the paint.

What to Look for With Old Cans

In addition to a puffed-up lid or bulging can, signs that paint is past its prime include a thick, rubberlike film topping it, or paint that doesn’t mix well when stirred or doesn't stay uniformly blended for 10 to 15 minutes after mixing. 

Even if your paint seems okay, it's a good idea to try it out on a piece of cardboard. You want to make sure that it can still be applied easily with a brush or roller, the color is uniform, and the surface is smooth and free of visible particles.

"Once you're ready to paint, use a brush or roller to blend the paint into what's already on the surface," de Paz says. "You want to go beyond the spot you're touching up." If the touched-up spot is noticeable when dry, try again. The last resort is to repaint the entire area.

What to Do With Leftover Paint

If it's good, save it. The best way to preserve latex paint is to store it in an airtight container to keep the water from evaporating and to prevent microorganisms from getting in. You can keep it in the original can or transfer the paint to a clean jar with a screw-on lid. Keep it at room temperature. That means don't store it in a garage or shed, where temperature extremes can ruin the paint. Label leftover paint—"Rob's bedroom walls, April 2017"—so that years from now you'll know where you used it.

If it's bad, dispose of it. It's important to properly dispose of unsalvageable old paint. To find out what to do where you live, check the website of your municipal or county government, or call your local department of public works. Go to Earth911 for more details on the disposal of leftover paint.

Looking to buy some new paint? Consumer Reports has tested major paint brands in dozens of finishes, and we report our findings in our ratings. Below, we highlight a few of our top picks in alphabetical order, not in order of rank, should you be heading to a home improvement store.

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Behr Premium Plus (Home Depot)

Price: $31

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Behr Premium Plus Ultra (Home Depot)

Price: $36

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Benjamin Moore Aura

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