extremely helpful…thank you! I stumbled on the first way, you showed me another way. What I am working my way towards is being able to do this in only small areas. Say If I had a flat chunk of steel with several spots of rust and/or corrosion sporadically located throughout the plan. With the manner shown I could have rust on one side and clean steel on the other…but not very realistic “per say”. How could I set this up so that these spaces of rust blend into the surrounding “clean steel”? I created an alpha channel in Photoshop and applied that with the attached results. Is this simply the only way of achieving this effect?

In this picture the alpha channel was the darker rust while the light area was the actual painted metal (needs work I know but this is for learning purposes!) Any suggestions or ideas? I guess I was hoping there was an internal method for being able to make all those spots and then blend into the parent (under) material…are these imported alphas simply the way to go?
Nice work by the way and thank you very much!!!!
J