I have a bit of experience in this area, through allot of trial and error I found a paint that when properly applied was very durable.
It is a spray paint though so it won't help with the issue of the hobby spray gun.
The paint is Krylon Fusion, it bonds to plastics on a molecular level.
The sheath below is my Tiger/Zebra stripe pattern using the above mentioned paint.
I happened to have just read a kydex manufacturer website suggesting the sherwin Williams industrial paint Prolane. This is supposed to be the same as dura coat.
http://www.kydex.com/Uploads/Files/...ief - Painting KYDEX® Thermoplastic Sheet.pdf