Q. We know that overlaid plywood forming panels have greater dimensional stability and smoother, more durable forming surfaces than nonoverlaid panels. But what factors should we consider when choosing between medium-density overlaid (MDO) and high-density overlaid (HDO) panels?
A. If you want the smoothest possible concrete finish and a high number of form reuses, choose HDO panels. Their hard, abrasion-resistant surfaces produce a glossy finish, and with reasonable care the panels provide 20 to 50 reuses. Some concrete forming specialists can achieve up to 200 reuses with careful maintenance of the forms. Concrete-forming-grade MDO panels create a smooth, matte concrete finish. If your job doesn't require a high-gloss finish or a large number of reuses, MDO panels may be the most economical choice. They may also be a better choice for jobs requiring forms curved to a radius less than 8 feet, because the lower overlay resin content produces a more flexible panel surface.
Bruce Cordova and Fulton Desler, "Selecting and Using Plywood Forming Panels," Concrete Construction, October 1995, pp. 817-822.