I wanted to learn to airbrush so I sprung for a Grex and invested some time experimenting. I am very pleased with the Grex and even more pleased with the results it creates - with one exception. For some reason, it will shoot Grimy Black for about 4-5 seconds before it clogs and stops spraying consistently. I am using acrylic Polly Scale. While this may not be the ideal airbrush paint, I have learned how to thin it and can get it to shoot GREAT - except for Grimy Black. It is readily available at the local hobby store in every color I need. I tried Badger MODELflex and get the same results. What I am primarily looking to do is to apply a Grimy Black overspray.

What's the secret?