Thanks everyone for the fast answers and pics yesterday. 
@Ares9323 answered my topic Question - best way to make a [fake] Bool in Mat code, that I can control from Character BP.
- (since there appears to be no Mat-Bool related node in Blueprint code?) So i’ll mark that as the solution for others to search.
- Everyone else note that this is a hard on/off (acts as a Bool) - no smooth transition between the effect change.
But @Ares9323’s question and @jwatte’s reply is interesting. I see it is best for what I want instead - a smooth blend from my default Material to the Rain effect.
