You mean that by wetting the back it adhere to the plastic board and the paper doesn't move?
I have seen my paper, also 140 lb, buckled when I painted large surface while being taped on the board but it returned flat when dried. I image that it would also depend on the size of the painting.
When you say wet on wet, does it mean you 'paint' water first and then add the pigment and move it around in the watered surface, or that you paint one color first and then add a second color before the first one is dry?
Thanks. New set of vocabulary to master with WC!