If you’ve ever worked with contact cement, you know that the glue you are using can get old and hard with time and exposure to oxygen. 
 
Here is a trick I use, to revive old hardening glue and not let it go to waste!
 
Whenever the glue in my spray gun starts to get thick, rather than letting it sit there and harden or settle, I pour the leftover glue into a mason jar. This way, I can save the glue for projects that require hand brushing. I recommend keeping a paintbrush in the mason jar with the glue, so it also doesn’t get dried out or harden. 
 
With time, even the glue in the mason jar will become thick and hard to spread, because of the oxygen reacting with the glue whenever you have the jar open. When the glue gets to this point, I will add a bit of acetone into the glue to thin it back out. Now, many people will advise against this, in fear that it will degrade the quality of the glue. However, if you look up the material safety data sheet, you will see there is acetone in the glue already when you buy it. I have been adding a splash of acetone to my glue whenever the glue gets thick for years, and I can assure you I have never had any projects come apart, or jobs returned due to degrading glue. 
 
When you do add some acetone to your glue, poor in just a small amount at a time to thin out your glue to the desired consistency. Make sure to mix in the acetone fully, because if you brush straight acetone onto your project, it will mess up the material you are applying the glue to.