MDO and HDO plywood

Overview

MDO (medium density overlay) and HDO (high density overlay) are plywood panels faced with a resin-treated fibre sheet that gives a smooth, paint-ready skin and hides the grain. Toledo describes MDO as plywood with a B face and back suitable for interior and exterior use, and says all edges should be sealed. APA lists MDO and HDO as optional face treatments for its Marine grade. HDO is the harder, glossier overlay used on concrete forms and heavy-duty signage.

Pros

• Paint-ready face with no grain telegraphing through, unlike CDX or A-C.
• Exterior glue; Toledo sells it for interior and exterior use.
• Stable and flat, with a wood core that holds screws.
• HDO resists abrasion and is reused: Toledo's Plyform HDO is rated for up to 20 pours of concrete.
• Available in 4x10 as well as 4x8.

Cons

• The overlay will not take stain, so it is paint only.
• Raw edges absorb water; Toledo says to seal all edges, and to re-seal cut edges on Plyform.
• Price varies widely: Toledo's 3/4 in MDO runs from $69.95 for a G1S domestic sheet to $173.95 for a 4x10 G2S.
• Overlay scratches and chips can telegraph through paint on thin sheets.
• Larger sheets are heavy.

Variants and subspecies

• MDO G1S / G2S: overlay on one or both faces. Toledo lists 3/4 in G2S import at $105.95 and domestic G1S B grade at $69.95.
• HDO (Plyform): overlay bonded to both sides with heat and pressure, dark brown film, phenolic, 100 percent moisture-resistant glue; Toledo's 3/4 in is $54.95. See Formply (concrete form plywood).
• APA Marine MDO/HDO: Marine plywood with overlaid faces, per APA; see Marine plywood.
• Exterior structural plywood: see Exterior and structural plywood (fir).

Working and finishing

Cut with a carbide blade, tape the cut line to keep the overlay from chipping, and seal the edges immediately. Prime with a bonding primer, then paint with exterior-grade enamel. Fill edge voids before painting; edge banding is not usual for sign work.

Staining and finishing colour

Not stainable: the overlay is a resin sheet. Colour comes from paint, and lacquer or enamel gives a smooth finish. The overlay is dark tan to brown, so light colours need a primer.

Sustainability and handling

MDO and HDO use exterior glue; the overlay uses phenolic resin on Plyform per Toledo. EPA excludes structural plywood from its formaldehyde rule (EPA FAQ, checked 2026-09-30), but overlay panels may not be structural-rated, so ask for the documentation. Overlaid sheets are heavy and edges chip, so carry on edge. Cut dust is irritating.

Popular projects

Outdoor signs and billboards, painted cabinet doors and panels, trade-show and display work, trailer and truck-body liners, concrete forms (HDO), and shop jigs.

Buying notes

Specify MDO or HDO, G1S or G2S (one or two faces), thickness and size. Toledo (September 2026): 1/2 in G2S unprimed $79.95; 3/8 in $71.95; 3/4 in G1S domestic $69.95; 3/4 in 4x10 G2S $173.95. Imports and domestic stock differ in price; compare. Seal edges on arrival.