Joe,
I am sure there products I don't know about, and this might one case where it highlights that point.
You mention :
I've been told that 3M makes a mask that can be washed off with water
I'd like to hear more about that material if it is available.
Back in the old days, they used to sell a product called "Masking
Mud" or something similar. It was heavily mixed with glycerin and could be hand painted onto a metal sign can. The
mud never dried, but you could then spray a layer of laquer over it. When the laquer dried, they just sprayed the remaining
mud off with water. That was a way of getting a good white letter with a solid red background. I believe Atkinson mentions the
mud recipe in his earlier book and then again in his Last and Final Book on Sign Painting...but when I heard about it, I always figured the brushed on mask would dry before spraying.
GripFlex or SprayLat actually does dry. It is a really cool product, though I haven't used it in a long time. It comes in either light blue or pink and can be cut with an X-Acto knife, then peeled. (As long as the underpaint is not porus. Needs to be solidly painted and smooth) It would work in this process, but you would need to cut around the edge of the letter carefully to hold the black reveal.
Let us all know if you find a water based mask that can be washed off afterwards.
Mike Jackson