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Why it is worth keeping that half-empty tin

There is a particular moment, usually on a Sunday afternoon with a roller still drying on the windowsill, when you look at the tin of emulsion you have just finished with and wonder whether to keep it. The temptation is to bin it and reclaim the shelf. Resist that urge. A well-stored tin of paint is one of the most useful things in a home, saving you a trip to the shop the next time a door frame gets scuffed, a child draws on the hallway wall, or a radiator pipe springs a small leak and takes a patch of ceiling with it.

The catch is that paint is a perishable product. Left open to the air, exposed to a freezing garage or a baking loft, it will skin over, separate, or turn into a lumpy sludge that no amount of stirring will rescue. The good news is that storing it properly takes only a few minutes and costs nothing. Here is how to do it well.

Label every tin before it goes on the shelf

In six months you will not remember which white is which. Trust me on this. Even if you are certain that the bedroom is the pale one and the landing is the other pale one, the tins will look identical and you will end up painting a test patch in the wrong shade and wondering why it does not match.

Write directly on the lid or on a strip of masking tape stuck across it, and include more than you think you need:

• The room and surface it was used on, for example "hallway walls" or "kitchen ceiling".
• The brand, finish and colour name, plus any code printed on the tin.
• The date you opened it, and the date you last used it.
• The base if you know it, such as water-based or solvent-based, because this affects how you revive it later.

A permanent marker works better than a biro, which smears on a damp lid. If you have several tins from the same room, number them and note the order you used them, so you can match a part-used tin to the right wall.

Seal the lid properly, not just firmly

Most paint fails in storage because air got in. A skin forms on the surface, and once that skin is stirred through, you have lumps for good. The seal matters more than the tightness of your grip.

Before you close the tin, wipe the rim and the groove clean with a rag. Dried paint on the rim is the single most common reason a lid will not sit flat, and it lets air in through a gap you cannot see. Press the lid down evenly around the edge, working your way round rather than thumping one spot. A rubber mallet or the flat of your hand is fine; a hammer will dent the lid and make the problem worse.

For extra insurance, lay a piece of cling film across the top of the tin before replacing the lid, or press a sheet of plastic directly onto the surface of the paint itself. This traps far less air against the paint and makes opening the tin next time much easier. Store the tin the right way up, never upside down, and never stack heavy tins on top of a part-used one.

Keep paint away from frost and extreme heat

This is where most stored paint quietly dies. Water-based paints, which covers most modern emulsion and a good deal of trim paint, are damaged by freezing. The water in the emulsion forms ice crystals, and when it thaws the binder no longer sits together properly. The paint may look fine in the tin but dry to a chalky, powdery finish, or simply refuse to adhere. Once that has happened, no amount of stirring will bring it back.

At the other end of the scale, sustained heat speeds up the curing and separation process. A loft that hits 40°C in July, or a cupboard beside a boiler, will shorten the life of a tin dramatically.

The ideal spot is a cool, dark, dry place that stays above freezing and below about 25°C. Suggestions that work in most UK homes:

• An understairs cupboard, as long as it is not damp.
• A bottom kitchen cupboard away from the oven and radiator.
• A garage or shed only if it is insulated, or if you move the tins indoors between October and April.
• A high shelf in a hallway cupboard, which is often the most stable spot in the house.

Solvent-based paints such as gloss, eggshell and wood stain cope with cold better than emulsion, but they are flammable, so keep them out of hot sheds and away from anything that could spark. Always store paint out of reach of children and pets, and never decant it into an unlabelled drinks bottle or food container.

Check before you use it again

When touch-up day arrives, do not simply shake the tin and go. Open it, look at it, and smell it. A little separation is normal, and water-based paint often benefits from a thorough stir with a flat stick rather than a shake, which just whips in air bubbles. Skim off a thin skin with a stick rather than stirring it in.

If the paint is smooth, uniform and free of lumps, it is almost certainly fine. If it has a sour, cheesy smell, if it has turned solid, or if it has been sitting in a frozen shed since last winter, it has had its day. Take it to your local household waste recycling centre, where paint is accepted for safe disposal. Tins that pass the test will give you a match you simply cannot buy off the shelf, and that is worth the few minutes it took to store them properly.