Use Polish Thinner to thin old polishes.

A few drops is all that needed, this will last years.

It is also great for nail art striping bottles.

TIP: Do not use Polish Remover to thin your polishes as it evapourates quickly and your polish will dry up in the bottle. Using a remover will make the polish look matt!

Dont forget to buy a plastic dropper and keep it specially for the thinners.