Yes, latex paint does expire! Latex paint does not, however, expire like milk or food does.
Paint can, and in most cases will, become unusable after some time in storage. How long this takes depends on how the paint was stored.
If properly handled, latex paint can be safely stored for several years. Conditions must be perfect for the paint to reach its maximum life.
Manufacturers know this. In fact, the storage life of paint varies so widely that the manufacturers avoid printing an expiration date on the can.
If age can't be used to evaluate paint, how can we tell if paint is still good?
How can I tell if the paint is still good?
To understand how to tell if paint is still good, we have to understand how paint dries. Latex paint contains paint particles suspended in solvent. When the paint is applied to a wall, the solvents evaporates leaving behind a coat of hard paint. The paint particles and the solvents do not necessarily mix well. When a paint can is left undisturbed for a while, the solvents and the paint particles will separate. That's why paint has to be stirred before use.
"If properly handled, latex paint can be safely stored for several years."
After years in storage this separation can become irreversible. The solvent evaporates, the paint is covered by a hard skin, or mold grows in the can. All these are signs that the paint has gone bad.
When you open the can you should be greeted by a chemical smell. The entire air space in the can should be filled with gases. If the paint smells rancid even a little bit, throw it out.
The solvents and the paint particles will inevitably show some signs of separation. In the case of latex paints this means there is a layer of water on the top of the can. That's fine as long as the paint returns to normal consistency after it has been stirred. If there are any signs of clumps or other irregularities, the paint has gone bad. It will probably stick to the wall, but it's anybody's guess how long it will stay there.
Paint that has been frozen is unusable. Its chemical properties have changed. A single freeze/thaw cycle is enough to ruin the can.
Whatever you do, do not use paint that shows signs of mold or smells even slightly bad. The results will look ok, but your entire place will smell like the paint does. After being freed from the can the smell will get stronger. And it won't go away easily.
Don't use latex paint if
- The paint is moldy
- The solvent has separated from the solids and the two don't mix
- There are clumps in the paint that don't separate when stirred
- The paint does not turn one consistent color after being stirred
- The paint went through a freeze/thaw cycle
- The paint smells rancid. A chemical odor is a good sign
- Otherwise you are probably OK
I know, this answer is not as clear as you want it to be. Knowing for certain requires a chemical analysis, which is just too expensive.
All this makes the best answer one you probably don't want to hear. Latex paint does expire. Manufacturers don't print an expiration date on the can, because the shelf live of one can to the next varies greatly. Use the chart above to determine if your paint is still usable.
The best strategy remains to be to buy only as much paint as you need. Avoid storing paint. If you have any doubt if your paint is still good or not, throw it out.
Paint is inexpensive compared to the work you have to do to get it on the walls. You don't want to do the work twice.