This is a very common question that we get. Here's a few simple tips:
If the paint tin rusts through, you lose the paint inside! So it's worth keeping the tin away from weather and excessive moisture. Try to avoid storing tins on damp floors.
If your tin starts rusting from the inside it's possible the internal lacquer of the tin has been damaged (this can be scratched if using sharp items when mixing). Decant into another container sooner rather than later, or risk losing that valuable paint.
Water based paints can be ruined by frost, so don't store them on the floor in your shed or garage, because it's likely to get around freezing point sometimes. Paint stored higher up tends to be kept in slightly warmer temperatures.
Avoid storing in overly warm places because this could result in internal expansion that could pop the lid off. Then you get mess and a solid tin of leftovers.
Ensure that the lid is firmly fitted and then you can actually turn paint containers upside down every 4 - 8 months. This reduces the risk of heavy settlement that can be extremely difficult to mix back in when you want to use it again.
Always mix any paint product thoroughly before use. If after mixing you're finding bits in it (probably old dry bits from the top), simply strain the paint using a fine sieve or even an old pair or tights! You might need to find a new container.
Large glass jars are suitable for storing small amounts of left over paint and they are often freely available. Don't forget to label it, and keep it out of reach, it's definitely not a carbonara sauce!
Correctly stored paint should be able to last for over 10 years, it's shame to simply throw old paint products away. If it's the wrong colour, then consider using it as a base or undercoat for something else.
We hope these simple tips help...