If you take a few seconds to wipe the neck of the nail polish bottle before replacing the top, you can create a tight enough seal to preserve the consistency of the formula. If you don't take care to secure your nail polish bottle carefully, over time, air can get to it and cause it to thicken. Fortunately, when you open your nail polish to find that it has changed into a goop-like mess, you can add something to it to dilute it.
Insert the end of a dropper into the bottle of enamel solvent or pour 1 tbsp. of solvent into a small container.
Gather the enamel solvent into the dropper. Open the bottle of nail polish and drop two or three drops of solvent into the bottle.
Close the lid securely and shake vigorously for 10 to 15 seconds. Open the bottle and check the consistency.
Add one or two more drops, replace the lid and shake vigorously if the polish is not diluted satisfactorily.
In a pinch, you can use nail polish remover instead of enamel solvent. Be aware that nail polish remover contains oil and fragrance, which may alter the fingernail polish beyond dilution.
Use a dropper to avoid adding too much enamel solvent to the polish.