We live in Beautiful Southern California Irving/Laguna Hills area where people like Tony choose to use raw MDF flexys. Besides, he glued the solid walnut to bare MDF and most likely clamped and glued them together. (By engineered substrate do you mean MDF, or that plastic bed liner?) Thanks for the tip and heads up though. I've built gates and worked with doors so I know how the Santa Ana winds and humidity levels can cause frustrations.