Panel Products - Surface Finishes

Surface finishes for wood-based products

A protective finish is generally necessary when using exterior grade wood-based panels in external situations, except perhaps for short-life non-construction uses, for example, site hoarding or boarding-up purposes.

Wood-based panel manufacturers recommend that any exterior grade panel used externally should be fully painted prior to installation - this includes both faces and importantly all edges, including those cut on-site. This protection will help to minimise moisture ingress and provide more balance to the boards, for example to prevent thinner boards, such as used for soffits, warping in service.

There are a variety of different paint finishes available and guidance on their suitability and use should be sought from the paint/coating manufacturer for the particular wood-based panel type and hazard class, although it is possible to obtain reasonable performance from a conventional paint finish on most panels. For plywood, a good face grade must be used, and BS EN 635-4 gives guidance on which surface grades are suitable for coating under each service class.

A surface grade I plywood that is sanded or textured, with a face veneer thickness of 0.4 mm to 3 mm, and inner plies a maximum of 5 mm with no open defects on the first inner ply, can be painted for use in service class 2 conditions.

It is essential that care is taken with surface preparation and the application of finishes on all boards intended for exterior use. Regular maintenance will help prolong the service life of wood-based panels when used in external applications.

Low build exterior wood stains should only be used with plywoods as they are not suitable for other board types. However stains possess certain advantages over film-forming finishes by being more able to cope with the behaviour of exposed plywood. Exterior wood stains will not prevent surface checking but are less likely to react to it by flaking off than paints. Redecoration with a pigmented product will protect the checked surface and should present an acceptable appearance. Patches, inserts, filler and overlaps on plywood face veneers will show up with a light coloured stain finish.

Note that some plywood/exterior finish combinations in Hazard Class 2 situations such as soffits or under eaves, can cause salts to migrate and appear on the surface of the coating. The use of light colours can make this effect unobtrusive.

Plywoods are available overlaid with specialist slip-resistant coatings which are used in such applications as; lorry decking, scaffold platforms, ramps and mezzanine floors. These types of plywood do not require further surface preparation or finishing, and an advantage of many overlaid board types is that factory sealing of edges is standard, but re-sealing of cut edges and drilled holes prior to installation is essential if the desired service life is to be achieved.

Other speciality plywoods are available with a resin-impregnated paper overlay for painting. The impregnated paper substrate provides a smooth defect free surface ideal for painting. Some end uses for this product are signboards, fascia panels, playground equipment and vehicle building.

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