Reply To: Plywood

There are two types of phenolic paper coated plywoods. That are used for different purposes.

The type Usually used for concrete for work is technically called high density overlay (HDO). The coating is designed to not stick to wet concrete and is fairly waterproof. It also means the slickness And smoothness Of the the surface is such that it doesn’t take paint or finishes well and it of limited utility for other purposes. It has more resin than MDO. The smooth surface of HDO gives a very smooth concrete surface, mimicking steel form work. It should be coated with a release agent when used for form work. Canola oil based products like modern chain saw bar oil work well and are environmentally sound.

Probably of more use for most work is Medium Density Overlay ply(MDO). It is designed for outdoor use, particularly as the material used for highway signs and comes with a matte finish that takes paint better. The overlay is available on one or both sides. PVA glue and epoxies will also stick to it. Years ago I built a clamming skiff with an MDO bottom and it held up well until I sold it 15 years later. I also built some planter boxes when I moved into my current house in 1993 and the planters, lined inside with a urethane roofing product, are still doing well. ( the cedar trim needs repainting, though.) I did also coat the horizontal top edge with the urethane, but the sides and bottom only have primer and paint.
While MDO can be used for form work, that isn’t its real purpose. it comes in several grades.
With both products, edges should be sealed with a primer, preferably oil based, or with a marine epoxy in particularly harsh environments.. that’s true for any ply.
And the phenolic resin makes both surfaces more durable than ordinary ply.