I'm sure many of us polishaholics have often come across a gluggy old bottle of polish and wanted to pour a whole bottle of remover in. But does it ruin your polish?
Unfortunately, there's no easy yes or no - the incredibly unsatisfying answer is: it depends. It depends on 2 things: the type of nail polish remover and the composition of your polish.
Polish goes gluggy over time because the solvent (wet stuff) slowly evaporates. Adding thinner replenishes the lost liquid, usually ethyl acetate or butyl acetate, two very similar non-polar solvents. Adding the first name on the ingredients list is the best bet for restoring the original consistency. But cheap thinners are hard to find.
There are thousands of different removers on the market. They are usually water and either acetone or ethyl acetate, sometimes with small amounts of colour or moisturiser added. So it makes sense that if your remover is pure ethyl acetate (rare, but possible) or the vast majority is ethyl acetate (hard to know if %s aren't listed), there will be no problem using that as a thinner.
However, if your polish remover has lots of water (very common, since water is cheap, and they can claim it's gentle since there will be less solvent to strip oil from your skin), the problem is that water doesn't mix very well with ethyl/butyl acetate. This means that for most polishes, the finish will end up less smooth once water has been added, and depending on how much you add, the incompatible solvents can result in a streaky look. This might not be a concern when it comes to bumpy glitters, or if you plan to use a topcoat, or if the polish had a dodgy formula to begin with.
Adding pure acetone is less likely to lead to incompatible mixtures since it mixes well with ethyl/butyl acetate, but it can still affect the composition of the polish and give a bumpy or dull finish. An additional concern with acetone is that it dissolves more things than ethyl/butyl acetate, so some glitters and shimmers which are solvent stable in the original polish might start leaching after acetone is added.
So there are a lot of factors to consider before you add remover to your polish! The best thing to do is to dump a small amount of goop onto a piece of foil, add a drop or two of remover, stir, and see how it dries.
The bottom line: Adding nail polish remover will generally change the finish of your polish, unless you're using pure ethyl acetate. This might not be a concern if you always use topcoat. The safest thing to do is to test a small amount first, or use a thinner that contains the top few solvents in your polish's ingredients list (usually ethyl acetate or butyl acetate).