4008 Restorer, a water-based, biodegradable cleaner used for soaking airbrush and spray-gun parts after painting. Can be reused as long as it is stored in a metal or ceramic container.
Restorer is very strong, so always try in a small amount. The solvent also dissolves softer plastics, so keep away from eg. O-rings that are not PTFE coated. If you used Restorer in your airbrush, always flush it with water afterwards too.