They are made to use inks or such stuff and any thicker material will stuck them ( how do I know ?) This considered, a wash or similar practice could be considered.
If you want to see it as a piece of art ( and we all deserve a piece of art ) look after the very old ones, with the green handle and different cup, as they are much better than the new ones ( and how do I know this also ?)
But the beast is very difficult to tame, and very air consuming ( by the way, the less air consuming airbrush I own is a Devilbiss Sprite, it's amazing how little this thing consumes, a real ecological airbrush, maybe) so it remains kind of curiosity anyway.