When finishes such as Permagloss and Varathane "crust over" on top, is a significant portion of the essential solids lost from the solution that remains?

I continue to use my Permagloss, for example, by breaking through the surface film that forms in the bottle and using the finish beneath it. Is this a mistake?

Thanks for any help.

I am guessing that the top layer would tend to seal off the lower liquid portion and protect it to some extent. Over time it would continue down until the whole bottle has solidified. You lose the solvent but no solids I'd think. But I'm no chemist.

I buy a bunch or metal sling shot loads (bearings) sized to fit through the neck of the bottle and keep adding them to keep the fluid level all the way up to the top of the bottle. To raise the fluid level in smaller incriments I use BB's. This shot can be reclaimed and used on the next bottle. I also save the very small flavor scent concentrate bottles that are usually sold by the cake baking supplies (Candles and such) to use when the fluid level gets real low. I couldn't tell you about the relative strength of the solution under the crust, I just do my best to mitigate it entirely, as the humidity in the air cures this product.
I eiliminate the possibility of entrapping humid air and then avoid any sub-crust potencey questions all togeather.