Some experiments using Allegorithmic Substance Designer.
Experiment 1 - Started off following a simple tutorial creating 2 basic materials (metal and rust), blending them to make a complex material and then viewing the results in Unreal engine using the Substance Plugin.
Starting with a basic physically based metallic roughness template and adding pattern and grunge nodes, a simple base metal material was created. The rust layer was then added and the node controlling the intensity of the rust exposed in the shader.
The Shader was imported into Unreal Engine using the substance plugin and it was easy to control the amount of rust due to the blend node exposed.