Thickness occurs from evaporation of the nail polish solvents over time, so this nail polish thinner replaces those lost solvents, and helps restore the desired viscosity.

This can happen if your bottle of Northern Nail polish gets down to about half full, has been stored in a warm/sunny area, or is over a year old. 

Use 1-3 drops of nail polish thinner when the nail polish becomes too thick. Add it drop by drop, so you don't add too much at one time. 

Be sure to store nail polish thinner away from sources of heat and flame, and out of reach of children and pets. Not to be used for any unintended uses: only to be used for dropping a few small drops into Northern Nail Polish to help thin the nail polish to desired consistency. Do not take internally, and wash skin in event of direct contact with nail polish thinner.