How long leftover paint keeps, and how to tell it has gone off

An unopened tin of waterborne paint kept somewhere cool, dry and above freezing will usually still be perfectly good several years later. Resene's own advice is simply that properly stored paint can last for years. Once a tin has been opened, stirred and part-used it ages far faster, and every reopening shortens what is left in it.

There is no useful expiry date on the label. The test is the smell, and whether it stirs back into a smooth even liquid.

What actually kills a tin of paint?

Four things, and they leave different evidence.

Bacterial spoilage. Waterborne paint contains water, and water grows things. Once the preservative package is exhausted, usually after repeated opening, bacteria get going and the paint develops a sour, rancid, faintly sewage-like smell. That smell is the single most reliable sign a tin is finished. Paint that smells wrong cannot be rescued and should not go on a wall.

Air. Every time the lid comes off, air goes in. Air skins the surface, thickens the body and eventually leaves you with a solid plug and a rim of dried paint that stops the lid seating properly next time. A half-full tin ages far faster than a full one, purely because of the volume of air above the paint.

Cold. Waterborne paint that freezes is usually finished. The emulsion breaks and the paint goes grainy or curdled, and no amount of stirring puts it back. This is a real risk in an unheated shed or an uninsulated garage in a Wellington winter, particularly out in the Hutt Valley where frosts are harder than in town.

Rust. Steel tins rust from the inside, especially around the rim where paint has been wiped. Rust flakes into the paint and comes out as brown specks in the finish, which you will not notice until they are on a white wall.

Solventborne enamel behaves differently. It does not spoil, so a well-sealed tin can sit for many years. It skins heavily and thickens, and it can be strained and thinned back into use much more readily than waterborne paint can.

How do you tell, tin by tin?

What you find	What it means	Use it?
A rubbery skin on top, liquid underneath	Normal air exposure	Yes. Lift the skin off whole, then stir and strain
Sour or rancid smell	Bacterial spoilage	No. Recycle it
Clear liquid on top, solids below	Ordinary settling	Yes, if it stirs back to one even colour
Grainy or curdled, like cottage cheese	Frozen at some point, or the emulsion has broken	No
Lumps that will not disperse when stirred	Partly dried, or gone off	Only after straining, and only somewhere it does not matter
Brown or orange specks through the paint	Rust from the tin	Strain it, and never on pale colours
Will not stir to a consistent colour	The binder has separated permanently	No

Stir it properly before deciding. Two or three minutes with a stick right to the bottom, not thirty seconds on the surface. A lot of tins written off as dead are just badly settled.

How do you store it so it lasts?

• Clean the rim before the lid goes on. Dried paint in the groove is why lids never seal again.
• Lay plastic wrap over the surface, press the lid down hard, then store the tin upside down. The paint itself seals the lid and keeps air out.
• Write the room and the colour code on the tin in permanent marker, not just on the label. Labels fade and tear, and a tin with no code is a tin you cannot reorder from.
• Keep it off a bare concrete floor and out of a roof space. Concrete draws cold and moisture into the base of the tin, and a roof cavity gets hot enough in summer to age the paint quickly.
• A cupboard inside the house beats a garage. If it has to be a garage, put it on a shelf, on the sheltered side, off the floor.

How much is worth keeping?

Less than you think. Half a litre per colour covers touch-ups for years, and it fits in a sealed glass jar that ages far better than a mostly empty four litre tin.

Keeping eight litres of a colour you will never use again is how garages end up with a shelf of solid tins that eventually have to be dealt with anyway. Decant what you need for touch-ups, label the jar, and take the rest to a recycler while it is still good enough for someone else to use.

Will old paint still match the wall?

Often not, and that is a separate reason not to hoard tins. The wall ages while the paint in the tin does not, so a sound tin of the original colour can still read differently against a surface that has had years of light and grime on it. Test it somewhere out of sight before you commit to using it on an interior touch-up.

How do you get rid of the dead tins?

Not down a drain and not into the gutter. Resene's instruction is blunt: never let waste water or solvents from washed equipment enter household or stormwater drains or sewers. Stormwater here discharges to the sea rather than to treatment, so paint tipped into a gully trap ends up in the harbour.

Resene's PaintWise scheme takes unwanted paint and paint containers at designated collection centres in their ColorShops. It accepts any brand of decorative paint, with a small charge on non-Resene products, and the tin has to arrive in its original correctly labelled container, sealed well enough not to leak. Good quality Resene paint is offered to community groups for reuse, solventborne paint goes to solvent recovery, and waterborne paint that cannot be reused is put to other uses such as graffiti abatement. Ring the shop before you turn up with a load, because there are limits on how much they can hold and a store cannot always take returns on the spot. A tin you know you will never use is better dropped off this year than kept for five more.

For small amounts of waterborne paint, Resene's own instruction is to leave it in its container with the lid off, out of reach of children, until the paint dries, and then bring it in. Solventborne paint is a different matter: Resene treats it as household hazardous waste, to be returned to PaintWise or held for a household hazardous waste collection day. Anything contaminated with lead-bearing scrapings takes the hazardous route as well, not the wheelie bin.

When is old paint fine to just use?

When it stirs smooth, smells like paint, and is going somewhere that does not have to be perfect. The inside of a garage, a fence, a shed, the back of a wardrobe, or as an undercoat under a proper finish coat. Old-but-sound paint is genuinely useful in those places, and using it up is better than paying to dispose of it.

What it should not be used for is a finish coat in a room you care about, or anywhere a straining pass and a stir cannot guarantee an even film.

If you are working out whether a room needs a touch-up or a proper repaint, we are happy to give you a straight answer. Get in touch.