How long will leftover interior paint remain usable?

How long will leftover interior paint remain usable? (I live in Bellevue).

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The short answer is: it depends. It depends on a number of factors, each of which can shorten or lengthen the life of unused paint. Let’s look at each factor separately:

Has the paint container been opened?

Paint BrushesOnce a can of paint has been opened, the paint begins to react with the air around it. When it was first added to the container by the paint manufacturer, it was done so in a temperature and moisture-controlled environment. Later, the paint can is opened, let’s say for argument sake, on a warm day in the middle of July. The first thing you’ll notice is, as you use a screwdriver to pop open the lid, there is a distinct ‘pop’, as the pressure inside and outside the can equalize. From that very second, the paint begins its ‘drying’ process. It’s definitely designed and manufactured to have an extensive shelf life – if for no other reason to allow sellers to hold on to it for longer and longer, and not to return it to the manufacturer – but once it’s opened, all bets are off.

How much of the container is paint, and how much is air?

You can all but eliminate the ‘air contamination’ problem if you remove all – or almost all – of the air before resealing the can. Perhaps you have combined the leftovers into a single jam jar, full to the brim, and secured it tightly. That will help a lot. But if the can is, as is often the case with leftovers, ten percent full and ninety per cent air, the paint will suffer when stored for a relatively short period of time. The high ratio of air to paint means there is a lot of air molecules available to react to the small amount of paint. Considering air is about eighty percent nitrogen, there’s plenty of that basic molecule to work the paint over time. I’m not a chemist, so I can’t say exactly what it will do to the paint, but the paint will not be the same six months later when you go looking for some to touch-up that scuffed corner of the living room. First of all, it will likely have formed a ‘skin’ on the top of the paint, something like the skin on a rice pudding if you leave it sitting around. That skin means that the chemical composition of the paint under it is altered, and does not have the same ratios as the original, full can of paint. What’s more, it might require ‘thinning’ with paint thinners even to be usable. By then, you can expect the color also to have changed because you can no longer be sure the tint has been preserved after the skin is removed and discarded, and the remaining paint is all that’s left.

Has the paint been stored in a cool, dry place?

Paint can withstand extreme cold and extreme heat when it has dried and perfectly set on an exterior wall, but while it’s sitting in a can, it’s in what is called its ‘volatile state’. It must be in such a volatile state because it must begin changing them moment it is put into action by being painted on a wall. That ‘power to change’, however, is also a necessary Achille’s heel, of sorts. You must take good care of it before it is used. That means keeping it above freezing point, but only within a relatively narrow temperature margin. And what’s more, the temperature fluctuations must be kept to a minimum. Paint does not do well when heated, and neither does it like to bounce every day from cold to hot. That ‘cool, dry place’ might be in the crawl space of your house, but only if your crawl space does not freeze, ever. Freezing is one of the worst things that can happen to paint. Mind you, the freezing point of paint is usually some degrees below the freezing point of plain water, but you can’t know for sure because each paint reacts in its own, unique way. For instance, water based paints will respond to cold in a different way to oil based paints. And as paint separates when left standing for long periods of time, some of it may have a different freezing point to other layers in the same can of now separated paint.

The risk to storing paint anywhere in your house is two-fold: If you have a fire and it reaches the paint, it can make a manageable fire turn into an inferno. Oil based paint in particular, burns hot and fast, and it’s a challenge to extinguish with what the fire department will try to put out your house fire with. The other risk is, a typical uninsulated garage can behave like a greenhouse in the summer, becoming much hotter than it is outside. A can of paint with only a half inch of paint in the bottom of it can overheat and even explode in flames. Paradoxically, a can of paint with very little paint in it can be more dangerous than a full one. A full can of paint takes longer to heat to a dangerous level, and does not have much air reacting with it to form a combustion cloud. Incidentally, this is how TWA flight 800 was brought down: a tiny amount of fuel in its center, almost empty tank became heated in the summer sun as the plane sat on the runway. Later, as it soared to 10,000ft, a tiny spark ignited the warm cloud of jet fuel vapors that made the perfect explosive combination.

My advice is to never store paint long term in your house.

Take note of the exact paint type and manufacturer in something like a “home project log book”. You can always go back to the store with that and in almost every case, they will be able to locate the exact paint you need. If you really must store paint for later, store it in one of those public storage facilities, or in a shed, well away from your house.

How long has it been since the paint was stored?

“Use it or lose it” is a good rule of thumb for paint storage. Paint is manufactured primarily to last a very long time on the walls of your house. If, however, you have stored it under ideal conditions for the whole time, by all means, use it up. You’re doing a great thing for the environment by not making your leftovers a problem for your garbage disposal service, and for the planet in general.

Whatever happens, remember that the safety of your family is your number one priority.

See you next week I hope!

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