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Marine plywood vs MDF

There is a big difference between Marine plywood and MDF in terms of quality, characteristics, costs and potential uses. This article compares these two different types of engineered wood boards.

MDF Marine grade plywood
'MDF' stands for Medium Density fibreboard, and it is different from plywood. Marine plywood is a superior quality plywood that is used in exterior applications.
MDF is made from very small particles of wood (fibres). Glue is added to these particles, and the mixture pressed together under high pressure and temperature to make these boards. Plywood is made by glueing together several layers of veneers (slices of wood) under heat and pressure. Rotary-cut veneers are obtained by rotating tree logs about their longitudinal axis, while a cutter peels off layers from the log.
The most common use of MDF is in the making of furniture for homes and offices. Marine plywood is most commonly used in applications where the ply is going to get a prolonged exposure to water. It is the material of choice for marine applications such as all the woodwork required for building ships and boats.
MDF looks better than marine ply. Most of the times its prelaminated with a decorative surface at the time of its manufacture. MDF looks better, but Marine grade plywood is basically meant for exterior applications such as boat-building, and not for building showy furniture.
These boards do not have knots or rings on the surface, since they are made from wood fibres. Plywood is made from slices of wood, and hence can have knots. This aspect is however controlled at the time of making plywood so that the plywood surfaces do not have any noticeable defects.
Most commonly used resin for making MDF is Urea Formaldehyde. (This is also the glue used in making Commercial MR grade plywood). The adhesive used in making Marine plywood, is unextended phenolic resin (phenol formaldehyde), which is a synthetic plastic resin. Unextended basically means undiluted.
The quality standards for MDF boards are specified in Indian Standards document IS:12406 (for plain MDF boards), and IS:14587 (for pre-laminated MDF boards) The Indian Standards for Marine ply are detailed in BIS document IS:710

Note that the shopkeepers in India, often refer to External BWR grade plywood as Marine ply. For them Commercial ply means MR grade, and Marine ply means BWR grade, (though strictly speaking that's inaccurate). You can check the links below for more information related to the various kinds of plywood.