I have made an earth-shattering discovery which, in the way of all my earth shattering discoveries, has undoubtedly been made by hundreds before me.
Art board is an excellent material for model building but it has flaws: it's very porous, if wet it expands and delaminates, and it tends to snag and tear. Solution: a thin coat of clear varnish. Simples! Except of course that the board then bends so... varnish both sides!
Suddenly I have boards which stay flat as I apply paint and scenic. I am happy. It also makes gluing much easier as the wetness of the glue does not cause the edges to pull paart or distort.