If nail polishes last forever.. then I wonder why they have the little shelf life thingie.. and also why it smells so sour after a couple of years later? I figure that it actually is bad and even though it might look safe to put on your fingers.. I mean, they don't put the shelf life on the bottle for nothing.
At the back of some OPI's and china glazes there is this symbol of what looks like a container with an open lid with a number (for my nail polishes it's either a 24 or 36) with an "m" beside it. That means that it's good for only 24 or 36 months, or 2 or 3 years. This symbol with the numbers are also at the back of most cosmetics (ie. smashbox, MAC). Here's a pic:
So.. I think that there's a reason they put these "Period After Opening" symbols behind nail polishes for a reason. Maybe the chemicals breakdown and can become toxic, I don't know. So I'm just going to be cautious and if my polishes turn old and funky, they will be chucked down the trash bin.
Well, if the thinner doesn't work, I highly suggest looking into Zoya's polish exchange if you haven't already. Sure beats throwing them out and getting nothing in return for it.
Yeah, I noticed OPI puts that period after opening symbol on their bottles (it's 24M in their case). I have no idea why they do that. Sometimes, I wonder if companies want us to believe that products have a certain shelf life, so we will toss them out and buy more.
http://www.scrangie.com/search/label/reviving old polish
Looks like you can keep it pretty much as long as you want!