Creating transitions between Materials

There are a number of ways to transition between two Materials, where a mesh changes from having one to having another. Some Material changes can be sudden, simply using the Set Material action in Kismet. Some transitions can be done using a linear opacity dissolve, where a parameter is animated to adjust a Lerp node's Alpha to weight from A towards B for instance. Others require a more organic blend though, such as a dry to wet transition through spattering rain drops, or a burning transition that crawls over a surface. The method explained here assumes that three Materials will be used: a Material set before the transition, another set during the transition, and another set afterward. The design of the blending is where the creative problem-solving lies, and there are numerous ways to come up with an interesting mask. Masking can reveal an underlying texture, or simple fade out to nothing. A lot of artists blend to transparent using noise textures...