I use Glassline paints in my fused glass work. Sometimes I can go for weeks or even months without using them. To help keep my paints fresh longer I use a small piece of Press’n Seal.
Unscrew the tip and place it over the opening then screw the top back on and tightly wrap the excess over the bottle. This creates a seal that helps keep air out. I also use this trick on my E6000 glue tubes. I either place a small piece under the cap or just over the whole top. It really makes a difference! When i do this, I don’t have to fish out the dried piece of glue from the tube before I use it.