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I find using nail polish thinners quite essential, regardless of how many nail polishes you many own. Nail polishes that dry out or become too goopy to use is something we've all encountered and it's never pleasant, especially when it happens to your favourite colour which -according to Murphy's/sod's Law- has also been discontinued and is hard to find. Users of Seche Vite topcoat also know -if they don't, they will- how important it is to use Seche Vite Restore in order to be able to use it past the first quarter of the bottle and not have to deal with excessive shrinkage of the polish or goopiness. Almost always though I hear that after a while the Thinner or the Seche Vite Restore is 'gone' even though you didn't use it much.
First let me explain why this is happening. Some of the substances in your nail polish are very volatile, which means that they evaporate really easily. Every time you open your polish bottle you 'lose' some of them as they escape from the bottle. Even when the nail polish bottle is closed, you still 'lose' some of them as no bottle is 100% airtight ( I mentioned a trick to make them more airtight here). Nail polish thinners contain these volatile substances, which is why they can restore your nail polish to its former glory without changing its consistency. Unfortunately, that also means that thinners evaporate very quickly and you're left with an empty bottle.
The solution to this problem is very simple: you should store your thinners in the freezer. The low temperature slows down the evaporation significantly to the point that you will not notice any change in quantity between uses. Don't worry, thinners are not like water; they will not turn to ice in the freezer. They will remain liquid and you'll be able to use them like before.
You can do the same for nail polish that you know you won't be using for a long time, but because it contains other substances, not just the volatile, it may feel a bit thicker when you take it out of the freezer; a few minutes at room temperature and a bit of a shake should fix it.
I hope this tip helped a bit. Here's a picture of a pretty fridge freezer to help you remember this next time you need to!