It sometimes happens that plywood used in boat construction tends to delaminate, so it is hoped that this article will point out some of the causes and methods of prevention of this trouble. It is based on correspondence with a timber technologist employed in the plywood industry.

Adhesives and Wood There are two main types of adhesive used in plywood manufacture, a urea-formaldehyde type similar to Aerolite and a phenol-formaldehyde type. They are easily distinguished as the former are usually whitish in colour and the latter reddish brown. When exposed to the elements without protection, urea-formaldehyde glues might last 3-5 years whereas phenolic glues will last as long as the timber itself assuming that the initial bond is good.

All plywood made to BS.1088 must be bonded with a phenolic adhesive, complying with the requirements of type W.B.P. - weather and boil proof - of BS.1203. Some foreign plywood sold to BS.1088 is only glued with a fortified U.F. resin. Other imported plywood has the correct adhesive but is made with a non-durable timber. BS.1088 specifies the adhesive, grade and species of timber to be used.

Protection The old sailing vessels used occasionally to have laminated masts glued with Scotch Glue which softens in water. The masts lasted for many years simply because the varnish was kept in perfect condition and the water kept out! In order to be on the safe side therefore, it is necessary to keep water out of plywood.

Water travels much more rapidly up the end grain that at right angles to the grain, hence the most important point to protect is the edge. Urea glues are not to be recommended for edge sealing because they get hard and crack so allowing the water to get in. One of the best water proofers is aluminium or a similar type of metallic paint, if the boat is to be painted. A more flexible sealer is a polysulphide rubber such as Polevomastic made by Evomastics Ltd., a subsidiary of the firm which makes Evostik. Varnish can also be used but not less than six coats are required and it is not as effective as metallic paint or synthetic rubber. Other proprietary sealers are available on the market.

Wooden mouldings are often used to protect the edge of plywood. If cracks are allowed to occur between the moulding and the plywood, water traps will be formed and if mouldings are used therefore, extra care must be taken to seal the end grain.

Resins at present little used by the amateur boat builder are resorcinol-formaldehyde resins. These are as durable as phenol formaldehyde resins and have the gap filling properties of urea-formaldehyde. Unfortunately the R.F. resins cost about seven times as much as the U. F. types.