Bacteria need certain conditions to grow, including darkness, moisture, and heat. All those conditions occur inside a closed paint can in your garage, shed, or basement. If you're working with a modern, low-VOC or zero-VOC latex paint, it's more likely to be ruined after a few years by bacteria growth.
In particular, an unopened can of latex or acrylic paint lasts up to 10 years. For unopened paints that are oil-based or alkyd-based, the shelf life could be up to 15 years. Additionally, chalk paint has a shorter lifespan from 1-3 years, and milk paint only lasts 1-2 weeks after its mixed.
A good rule of thumb with paint is, if you open it and it looks / feels fine (like when you stir it up) then it likely is fine.
Water-based paint: 4 – 24 hours
Water-based paints have a relatively short open-air time. That means it will harden quicker than most other types of paint. So, you should either pour it back into the paint cans or store it with an airtight lid as soon as possible.
"Paint that is beyond its shelf life can develop a rancid, sometimes rotten-egg odor," says Edrosa. If you open a can and find either of these problems, dispose of the paint and buy new.
You might notice the consistency of your paint has changed. There may be a “skin” on the surface or lumps that won't stir out. If either of these is present, your paint has most likely expired.
Signs of damage: If the paint has gone bad, it will look curdled like cottage cheese or overly thick like a pudding. If you dig out an old can of paint and you have either of these, it's best not to use the paint and dispose of it properly.
If it smells bad, bypass using the paint entirely — that smell is bacterial growth that will likely persist once it's on the wall. Small flecks or a thicker consistency, however, can be ignored for quick jobs if you're not a perfectionist.
Never store paint outside or in areas that are not temperature controlled.
For paint trays, use the heavy plastic kind, brushing excess paint back into the can, and letting the final layer of paint dry instead of rinsing out. The best place to clean brushes is in a bucket of water and finish with a clean-water rinse before shaking them out.
We've got the scoop on why old paint smells so bad and it is simply a buildup of bacteria and mould that causes it to smell like sour milk (OR MUCH WORSE). Bacteria sometimes gets into the paint from the manufacturer but it is most likely to becomes contaminated once opened.
All paint should be kept out of direct sunlight and in a place that's temperature controlled, ideally between 60 and 80 degrees Fahrenheit. While the garage is a popular spot to stow cans away, paint can freeze so reconsider if you experience harsh winters where you live.
Paint should be stored out of reach of children and away from food and drink. Most types of leftover latex paint, including AURA®, REGAL® and ben®, should be stored either in a lined metal can to prevent rust, or a glass or plastic container.
Paint Shake and Re-Tints
If you bring your old paint to a Home Depot paint center, you can have it shaken up in their machines. This saves the effort of stirring up your old paint, but it also sets up another great service: paint re-tints.
If the old paint is flaking, bubbling, or peeling off, it might not be a good idea to paint over it directly. This is because the new layer of paint might not adhere properly to the surface, leading to an unattractive and uneven finish.
What to Look for With Old Cans of Paint. 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.
Old paint is likely to separate. This is also a good opportunity to check the consistency of the paint. If, after stirring the paint, the consistency is chunky, lumpy, or has a cottage cheese-like texture, the paint has expired.
The bottom line is that watery paint is unusable. So if your paint is too watery and you attempt to use it to paint your home, you will notice that the paint won't adhere to surface correctly. It will be 'thin' and therefore will perform poorer than expected.
Chemical or paint smells
If you haven't recently used chemicals or paint, a strong chemical smell could indicate a gas leak, particularly from carbon monoxide. A distinctly chemical odor can result from a crack in the heat exchanger component of your furnace.
Make dried paints liquid again with water
As acrylic paints are water-based, you can make these paints liquid again by adding water. Hot water is best for this, which you pour drop by drop into the pot. Then stir the dried paint thoroughly and wait for the water and dried acrylic paint to mix thoroughly.
Add water. Dip your paintbrush in a cup of clean water and then gently tap the water from your brush onto the paint. Carefully work the water into the paint using your brush, adding more water if necessary, until the paint becomes more liquid.
Oil based paints have a shelf life of anywhere from 2 to 15 years. Depending how well it was sealed and stored. One of the best ways to tell if your stored paint has gone bad is to test it on a piece of board or timber. If lumping occurs it could be that chunky sized bits of paint have formed despite a vigorous stir.