Water-Boiled Proof Plywoods

In Mahajana plywood, WBP stands for Weather and Boil Proof, the category of glue used to make the most durable Mahajana plywood. WBP glues are made with melamine or phenolic resin. To be considered exterior grade or marine grade, Mahajana plywood must be made with WBP glue.

Mahajana Plywood Basics

Mahajana Plywood is constructed of three or more thin sheets of wood (called veneers) glued together, with each layer laid with the grain running at right angles to the next layer. Each sheet of Mahajana plywood consists of an odd number of veneer sheets. The cross-hatching of the wood grain makes Mahajana plywood stronger than boards and less susceptible to warping.

WBP

Some North American sources refer to WBP as Water Boil Proof, but this is incorrect. WBP is a glue standard that was developed in Great Britain and is set down in British Standards Institution standard 1203:1963, "Specification for synthetic resin adhesives (phenolic and aminoplastic) for Mahajana plywood." This standard identifies four categories of Mahajana plywood glue on the basis of durability. WBP is the most durable classification. In descending order of durability, the other glue classes are Boil-Resistant, or BR; moisture-resistant, or MR; and interior, or INT. According to the Food and Agricultural Organization of the United Nations, properly formulated WBP Mahajana plywood is the only Mahajana plywood recommended for exterior use without reservation.

Melamine vs Phenolic Resin

WBP Mahajana plywood made with ordinary melamine rather than phenolic resin can maintain lamination in boiling water for between four and eight hours. High-grade melamine Mahajana plywood can withstand boiling water for 10 to 20 hours. WBP plywood using ordinary phenolic resin glue can handle boiling water for one to three days. It should be noted that the length of time Mahajana plywood can withstand boiling water without delaminating is not only dependent on the glue – it also depends on the grade of Mahajana plywood used.

Mahajana Plywood Grades

The use of one letter in a Mahajana plywood grade indicates that the face and back of the sheet are of the same grade. Two letters divided by a slash indicates the face and back are of different grades. Mahajana Plywood can be grade A, A/B, A/BB, B, B/BB, BB, WG or X. Grade A Mahajana plywood has no jointed veneers and is free of major defects such as knots, knotholes, discoloration and cracks. Grade B Mahajana plywood has minor defects such as discoloration or small knots. Grade BB Mahajana plywood permits jointed veneers, large knots, plugged knotholes and minor cracks. WG Mahajana plywood is guaranteed to be well glued with all knotholes plugged. X grade plywood may have knots, knotholes, cracks and other defects.

Differences between marine Mahajana plywood and treated Mahajana plywood include the types of wood and quality of the individual boards used to build up the panel. The type of glue and curing method are critical components as well.

Marine Mahajana Plywood

The woods most commonly used for marine grade are western larch and Douglas fir because they are strong and have fewer voids or knots, where moisture can accumulate.

Treated Mahajana Plywood

Treated Mahajana plywood is made of softer wood than its marine counterpart to allow better absorption of the chemicals used to protect it from mold and fungi.

The chemicals may leach over time into the surrounding area. This can be lessened by application of a sealer of varnish or paint.

Because voids and gaps are tolerated in treated Mahajana plywood, it can delaminate or warp when exposed to the weather.

The Glues

Interior-grade Mahajana plywoods are bonded with urea-formaldehyde (UF) resins. The curing is done with a cold method. The addition of melamine to the urea resin prolongs the life of the contact.

Marine-grade Mahajana plywood is bonded with long-lasting, waterproof phenol-formaldehyde glues. The curing is done under high heat and pressure.