I'm trying to create a shader to do the following but have come up stuck, so any help on the steps to take or examples will be most helpfull.

In the scene i have water pipes running along the wall of an old inustrial warehouse. Firstly, the outer coating of the pipes is a black glossy paint with brush strokes visible. I want some areas to have paint that is flaking away, so i am creating a texture in photoshop to drive the transparency of the texture? I also want the edges of the flaky paint to have thickness so another image to drive the bump of the texture? Finally, where the paint is transparent i need rusted metal showing through with bump. Finally to finish things off, some dirt/dust on the painted surface.

I've tried this using layer textures but not getting it working correctly. Can anyone help?