Exterior grade plywood (WBP - Weather and Boil Proof) is specially made using a water-resistant adhesive to withstand a certain amount of moisture and can be used for outdoor constructions - sheds etc. and is sometimes used as a cladding material, particularly for insert panels under windows. WBP does require additional protection (paint or varnish) to protect the outer veneer.
Internal plywood is of a similar quality as Exterior grade but it does not use water resistant adhesive. It can be used for wall panelling, flooring and furniture.
Shuttering Ply is used in the construction industry for making shuttering boxes for containing concrete while it goes off. Although water resistance to a degree, the sides of this material are not finished with a decorative veneer and is generally not suitable for use where a quality finish is required.
Marine Plywood is made with waterproof adhesive so that it will stand immersion in water, the veneers themselves will not last forever under water so the material should still be finished with paint or varnish.
Plywood is normally available in 2440 x 1220 sheets (or subdivisions) and in thicknesses from 3 to 35 mm.
Working with plywood
If the sheets are to be used in a centrally heated room, store them in similar conditions before use so that the moisture content of the sheets will stabilise before installation - this will avoid shrinkage later.
Cutting
To avoid damage to the finishing face, cut into the panel from that face - for hand sawing have the face upwards, for power sawing have the face downwards. To reduce the damage, score through the outer veneer on both sides of the sheet using a sharp knife before starting to saw.
Thin sheets (up to 3mm) can be cut using a sharp knife.
From 3 to 6mm use a tenon saw.
From 6 to 12mm thick, use a fine tooth panel saw.
For thicker sheets use a coarse tooth panel saw.
Power saws can be used with thicker material but are more likely to damage the outer skin where the saw cuts away from the material.
Fixing
Plywood may be fixed in place with adhesive and screws or nails.
Generally only small screws grip well in thick plywood itself but even then they will not stand up to repeated changes in force. When fitting hinges to any thickness, use bolts with large washers and nut to 'squeeze' the sheet. It is not practical to screw or nail into the edge of plywood.
Where a corner joint is required, fit a softwood batten in the corner so than both sheets of ply can be fixed to it.
For thin material, always fix plywood to a pre-made frame, use glue and panel pins (on about a 200mm pitch), use a punch to knock the head of the pins below the surface. The frame should support all edges of each plywood sheet and at about 400mm centres (for 6mm ply), 600mm centres (for 9mm) or 900mm centres for thicker material.
Knocks to the edge of ply can cause damage to all the veneers which is hard to rectify, so fit a thin strip of softwood along the finished edge or put a rebate in the frame so that the edge of the plywood can be recessed. This will also improve the appearance as the edge of plywood is not very attractive.
When gluing plywood, roughen the surface with coarse abrasive paper and brush clean before applying the glue. Spread the glue evenly and apply pressure.
Finishes
Plywood which is supplied with a decorative veneer can be varnished or painted but to get a really smooth surface will need good preparation as most plywoods are not as smooth as planed timber.
