Man-made board

page 2

See our first article for general comments on man-made boards.

Hardboard

Hardboard is a compressed, composite board. Fibre residuals are saturated in a wet process and then compressed to a sheet. A fine fibre overlay is applied (normally to one side) to provide smooth face.

Three basic types are available - standard, medium and oil-tempered. A number of specially finished boards are available for specific purposes.

• Standard
  With a smooth surface on one side and a mesh texture on the other, this can be used for wall and ceiling panelling, floors, door panels, built-in cupboards and fitments etc.
• Medium
  This is less dense than standard hardboard but it is thicker and so more rigid. There are two types: LM (low density) which will take drawing pins and can be used for pin boards and notice boards, and HM (high density) which is suitable for wall and ceiling lining, partitions.
• Tempered Treated
  This is usually treated with oil to give extra strength and water resistance, it is particularly suitable for exterior use.

As well as being supplied in flat sheet form, hardboard is also available with various designs of perforations (to act as peg board or decorative grills) and with 3D decorative designs on one face (usually ridges or similar repeat patterns) although this latter type is increasingly hard to find.

Working with hardboard

When handling hardboard, take care not to damage the surface of the finish face. If a full sheet twists it can tear across the whole width.

Conditioning

To prevent sheets buckling due to a change in moisture content, manufacturers recommend they should be conditioned before use. There are two ways of doing this.

• For standard and LM (low density) type medium and tempered boards, scrub the backs with water, using I litre to every 2440 x 1220mm sheet. Then stack the sheets flat, back to back, for 24-48 hours (48-72 hours for tempered boards).
• For HM (high-density) type, medium boards and all boards to be used in centrally heated surroundings, stack the sheets on edge in the room where they are to be used, separating them with wood off cuts to allow the air to circulate. Leave HM medium boards for 48 hours and other boards for 72 hours before using them.

Use a fine tooth saw and cut into the face of the board, supporting it on both sides of the cutting line. Pre-decorated and plastic-covered boards should first be scored along the cutting line using a sharp knife and straightedge to prevent the edges chipping.

Fixing

Hardboard may be fixed by screws, hardboard pins, nails or adhesives; follow the manufacturer's recommendations relating to the particular type of board and its use. The heads of hardboard pins are designed so that they 'disappear' into the surface after they have been hammered home.

You can use general purpose impact adhesive (for instant fixing) and woodworking PVA or synthetic resin-based ones - where possible, pin or cramp the work piece while the glue is setting. Generally the glue only bonds to the top surface of hardboard and if a joint fails, it is generally not the glue but the top surface of the board which separates from the underlying board.

Finishes

Knocks to the edge of hardboard will cause damage, so fit a thin strip of softwood along the finished edge or put a rebate in the support frame so that the hardboard can be recessed.

Hardboard which is not already primed should be treated with special hardboard primer/ sealer before being painted or papered. Diluted emulsion paint (one part of water to four of paint) or aluminium paint may be used as a primer unless the surface is to be papered. Wood primer should not be used.

Hardboard is normally available in 2440 x 1220 sheets (or subdivisions) and in thicknesses from 2.4 to 13 mm.

MDF (Medium Density Fibreboard)

MDF is a type of hardboard which is made from wood fibres glued under heat and pressure. MDF has many qualities that make it an ideal alternative to plywood or chipboard. It is dense, flat, stiff, has no knots and is easily machined. Its fine particles provide a material without a recognisable "grain". Unlike plywood and blockboard, MDF contains no internal voids, and will produce better edges providing that it is correctly machined.

But use MDF with care - it is manufactured using urea-formaldehyde resin which maybe released in low concentrations over a long period of time. Formaldehyde can cause temporary eye and respiratory irritation, it can also aggravate respiratory conditions or allergies in some people. Proper ventilation and precautions during working and use will reduce the risk of such problems. Some people have been reported as being affected by just a piece of MDF in a room.

Working with MDF

Dust is a major hazard when working with MDF. The large amounts of dust generated by cutting and machining requires the use of proper breathing and eye protection. As a minimum, always use a dust mask while there is dust in the air, not just while actually cutting. A professional style respirator is preferable. Goggles should always be worn while using tools on MDF.

MDF can be cut in the same manner as a fine-grained hardwood although as it does not have a grain, it can be successfully sawn in any direction. MDF can also be machined, drilled and tapped.

Due to its high glue content, steel cutting tools will dull very quickly; the use of carbide tools is preferred.

Being without a grain, cutting or machining MDF will result in a very smooth edge providing that the tools are sharp and used correctly.

Fixing

Screws and nails can be used to fix MDF, however with these fixings, there is a risk of the MDF splitting if care is not taken. The risk of splitting depends upon the thickness of material, generally fixings should not be less than 25mm from the edge.

When using nails, drive them in at right angle to the surface, this will avoid bending and reduce the chances of splitting. Use ring-shank nails to ensure a good grip.

With screws, always use pilot holes - about 85 % to 90 % of the root diameter of the screw being used and at least as deep as the screw. Straight sheet metal screws with constant size shank give best results, avoid using tapered wood screws.

Because MDF can be milled to just about any profile, many of the traditional woodworking joints can be used as can dowels.

MDF can be glued using gap-filling or PVA glues. Epoxy and hot melt glues are also suitable. The edges of pieces of MDF can be glued to make larger sheets.

Finishes

As mentioned previously, MDF is typically made using urea-formaldehyde resin which will be gradually released into the atmosphere wherever MDF is used. While this will not affect most people, some people are sensitive to formaldehyde emissions so precautions must be taken. Where possible, use low formaldehyde or formaldehyde-free MDF, or consider methods of controlling these emissions through proper finishing.

Where a finish is applied, it should be applied to ALL exposed surfaces, even those not immediately visible. Finishes that work best at controlling formaldehyde emissions are solid surfaces such as laminates, vinyl covering, and finished wood veneers. Oil based paints and varnishes can be used. Less effective at controlling emissions are simple wax and oil coatings.

Stirling Board

Sterling board is a wood-based panel manufactured from softwood strands compressed and bonded together with exterior grade, water resistant resins. It is readily identified by the random pattern of flattened, softwood strands which make up the surface.

It can be used in place of other sheet materials but due to the surface finish, it is generally restricted to areas where appearance does not matter. Tongue and Grooved is also available for use on flat roof decking and flooring applications.

Working with Stirling Board

Stirling Board can be sawn using either a hand or power saw. No matter how carefully it is cut, the edge will tend to 'lift' on the reverse side. Always make all cuts of a sheet from the same side, then all of the delamination will be on one side.

Fixing

When fixing with nails, use ring shank nails to increase the grip. When using screws, there is generally no need to countersink the holes, as the screw head will compress the board as it is tightened.

Where Stirling board is to be used for roofing:

1. All edges should be supported on joists or other support.
2. Panel edges should bear approx. 20mm onto joists.
3. Nailing should be at least 10mm from the panel edges.
4. Panels should be fixed using approx. 3mm ring shank nails or screws whose length are 2.5 times the thickness of the panel.

Finishes

The surface of the ordinary quality is often considered unsuitable for a finished surface, the strands of wood often lifts slightly and, if pulled, will 'run'. The edge of the panels can become 'flaky' and are easily damaged by careless handling and knocks. This does not matter too much where appearance is not important or where a physical covering (e.g. roofing felt) is to be fixed to it.

Stirling board can be painted. Where a fine finish is required, factory sanded panels are available. With unsanded panels, the surfaces can be wire brushed to remove any loose wafers and resin deposits. Priming and top coating with oil based timber paint can give a good, although not perfect, finish. Water-based products should be avoided as they may cause some swelling of the surface wafers.