We've all been there. You head to your polish stash, only to find that bright pink you wanted to use for your pedicure is thick and clumpy. Or that the new polish you just bought is a bit to thick to apply easily.
A quick Google search reveals a TON of posts about what to do. What do they all say to do? Add a few drops of nail polish remover or acetone. Never, ever do this! The remover is exactly the wrong thing to use. What you need is nail polish thinner.
Nail polish thinner contains chemicals typically used as solvents in nail polish. These chemical hold everything, the pigments and the resins that hold the polish together. The solvents must also be carefully chosen to control drying time and prevent bubbling and peeling of the polish. A quick glance at the ingredients in Beauty Secrets Thinner (the house brand at Sally Beauty Supply, I've purchased this thinner a few times), shows typical solvents such as Butyl Acetate.
Nail polish removers such as acetone do a great job of re-suspending the nail polish ingredients during removal. If they didn't do this you would never be able to get the polish off. However, because they dry so quickly, the polish is not able to properly set up on the nail. So, they will thin out the polish, but what is the overall effect? The polish's drying time is off and even if you can get it to apply properly the look and wear time of the polish will be off.
So, if you need to thin your polish, go for a thinner closer to the original solvents used in your polish. An actual thinner only costs a few bucks and will actually save that bottle of polish. A few drops at a time, shake and add some more until you reach your desired effect. I've saved bottles that appeared completely (and I mean completely) dried out by adding thinner, shaking, and sometime letting the bottle sit for a week or so before moving on to the next step.