Substrates for veneer and intarsia

Why the choice of substrate decides the quality of the intarsia

In veneer intarsia the substrate is invisible, but not without consequences. Every veneer moves with its substrate: it takes on its flatness or unevenness, follows its movement with moisture, and cures in the geometry the substrate gives it under pressure. A substrate that cups after pressing takes the veneer with it. A substrate with an uneven surface hardness creates uneven glue bonds.

In geometric intarsia this effect is magnified by the exactness of the pattern. A geometrically precise veneer picture on a warped substrate looks distorted as soon as the finished object lies on a flat surface. Straight lines in the veneer no longer run straight if the substrate underneath has a slight bulge. Transitions of contrast that are exact in the veneer look visually restless on a surface that is not flat.

So the choice of substrate is not a minor decision about material but part of designing the process, alongside planning the pattern, cutting and assembly.

MDF: stability, flatness and gluability

MDF (medium-density fibreboard) is the most used substrate in veneer intarsia and in industrial veneering in general. Its main property is that it has no grain direction. It shrinks and swells with changes in humidity equally and predictably in all directions in the plane; the movement is much greater only in thickness. There is no risk of cracking or splitting along a main direction as with solid wood.

The surface density of MDF is high and even. That means good adhesion over the whole area with no porous zones or hard knots. PVA glue holds directly on the MDF surface, with no primer or pre-treatment. The edges of MDF are soft and can be sanded, but they resist tear-out poorly in milling or sawing; a cut edge of MDF is less robust than one of plywood.

The weaknesses of MDF are weight and absorbing moisture. MDF is heavier than plywood of the same thickness and takes up moisture noticeably at open cut edges. Edges have to be sealed if the object may be exposed to moisture. In closed rooms with stable conditions, that is no pressing limitation.

Birch plywood: lightness, strength and edge quality

Birch plywood consists of layers of veneer glued crosswise. The cross-ply structure spreads the wood's movement in both directions, so plywood distorts much less than solid wood with changes in humidity, but varies a little more than MDF. The birch face ply is dense and takes glue well.

The weight advantage over MDF matters in large formats: a twelve-millimetre birch board is lighter than an MDF board of the same thickness. For small precision objects weight plays a smaller role, but for wall panels or larger decorative boards, plywood is the weight-saving choice. The edge quality of birch plywood is better than MDF in sawing and milling: the glued plies keep tear-out at the cut edge smaller.

One risk with plywood is faults in the inner plies: cheaper kinds have voids or knot holes in the inner layers. These cannot be seen from outside, but can lead to loss of pressure over hollow spots during pressing. For veneering, a quality with selected face plies and gap-free inner plies, as good birch multi-ply has, pays off; cheap construction plywood is unsuitable.

Solid wood as a substrate: risks and exceptions

Solid wood as a substrate for veneer intarsia is the special case, not the standard. Solid wood has a pronounced grain direction and moves many times more across the grain than along it when humidity changes. A solid board that takes up moisture in summer swells across the grain and can put the veneer under tension. In winter, when heating draws moisture from the air, it shrinks again. These changes in size pass straight on to the veneer.

For geometric intarsia this difference in movement is especially problematic. A veneer assembly of several species, each reacting differently to moisture, lies on a solid substrate that moves on its own. The sum of these movements creates tensions that can open joints, tire the glue or lift the veneer.

Exceptions exist where the solid substrate is structurally necessary, for instance with solid frames into which intarsia inlays are set. In these cases the grain direction of the substrate has to be matched with the main direction of movement of the veneer assembly. Solid wood with veneer on it is only stable in the long run if freedom of movement is built in, or if the object is so small that the absolute changes in size stay below what the bond can tolerate.

HDF and thin boards for small objects

HDF (high-density fibreboard) is denser and harder than MDF of the same thickness. The higher density improves stiffness in bending, which matters especially when thin boards are used without a supporting frame. A four-millimetre HDF board resists cupping better than a four-millimetre MDF board.

For small intarsia objects, such as pen holders, card stands or trays with veneered surfaces, HDF allows wall thicknesses that would be too flexible in MDF. The surface quality of HDF is higher than MDF's, which leaves adhesion on the face just as good but makes sanding the edge a little harder.

In laser cutting HDF behaves much like MDF, but needs a little more power per millimetre, because its higher density needs more energy per area cut. The cut edge of HDF is more even and shows less fraying. HDF is therefore a good choice for laser-cut joining parts and inlay pieces, where the quality of the edge shows directly.

Composite boards and special substrates

Composite boards combine two different layers of material. Common examples are boards with an aluminium core and wooden faces, aluminium honeycomb panels with an MDF face, or plastic boards with a wood veneer face. These materials are much stiffer in bending for little weight, but are rarely the main substrate in a craft intarsia workshop.

They matter as special substrates where weight is a design feature, for instance thin hanging wall objects for which an ordinary MDF board would be too heavy. How well they take glue depends heavily on the face: wood-based faces take PVA directly, while plastic faces need a primer or a special glue.

In laser cutting, composite boards containing metal cannot be cut with standard CO2 lasers. If you use a CO2 laser, you are limited to wood-based substrates. CNC milling can cut composite boards, but needs specific tools for aluminium and plastic that differ from woodworking tools. Details of how suitable each substrate is for laser and CNC follow in the section below.

Moisture stability and the risk of distortion

All wood-based substrates react to moisture. MDF and HDF react evenly in the plane, which prevents cupping but does not rule out changes in size. At exposed cut edges MDF takes up moisture especially quickly and can swell. Edges that are not veneered or sealed are the most sensitive zone.

Birch plywood is more stable than MDF under moderate changes of humidity and changes size mainly in thickness, less in length and width. That is an advantage for veneer faces held in length and width: the cross-ply structure keeps the surface stable in both directions.

The critical quantity is not the absolute amount of moisture but the speed of change. A board that gets damper or drier evenly moves evenly and holds its bond. A board that moves differently on one side than the other through one-sided exposure, steam from below, heating air from above, cups. That is why intarsia objects should be kept in balanced conditions and not exposed to sources of moisture on one side.

For the choice of glue that means: more creep-resistant glues such as urea-formaldehyde glue are better suited when the substrate is exposed to movement with moisture, because PVA can give way gradually under lasting tension through cycles of humidity.

Flatness as the precondition for gluing

A veneered panel is only as flat as the substrate it cured on. The pressure forces the veneer into the shape of the substrate; if the substrate is bowed, the finished surface is bowed. Cauls of MDF or steel can even out small unevenness in the substrate, but cannot correct fundamental cupping.

Before every gluing, the substrate is checked with a steel straightedge or a precision rule. The straightedge is run over the board in several directions: lengthwise, across and diagonally. Every gap of light between straightedge and board shows unevenness. Boards with more than 0.3 millimetres of bow per running metre should be flattened, replaced or ruled out as substrates.

Newly delivered boards can already be warped from poor storage: standing upright, on an uneven base or in damp conditions. MDF and plywood are stored lying flat on an even base, to prevent sagging under their own weight. This detail has a direct effect on the quality of the veneer work applied to them later.

How well the substrate surfaces take glue

The surface of the substrate has to be able to take up the glue. MDF and birch plywood can be glued with PVA without any pre-treatment. The surface is absorbent enough for the glue to penetrate and form a mechanical bond without running too much.

Treated boards, with primer, oil, wax, lacquer or release agents containing silicone, lose their ability to take glue. Residue of release agent on the substrate, transferred from a caul, can cause local glue failures that only show in sanding as bubbles or lifting spots. Press plates and cauls treated with release agent must not lie directly on the substrate; a release film or a sheet of paper as an intermediate layer is necessary.

The moisture brought in by water-based glue can make MDF swell briefly. This effect can be controlled if the glue is applied thinly and the pressure goes on straight afterwards. More on amounts of glue and how to apply it is on the glues page.

Board thickness and stiffness by type of object

The right board thickness depends on the type of object, the construction and the weight of the material. No universal minimum thickness applies to every use; the requirement follows from the task.

For desk objects built as a carcass, such as pen holders and storage objects, where the board serves as a wall, six to ten millimetres of MDF or plywood is usual. Below that, stiffness under load and tolerance for sanding start to become a problem. Above it, the weight of the object rises noticeably.

For inlays that are set into a frame and supported there, four millimetres of HDF can be enough. For larger wall panels without a frame, a thicker board makes sense, depending on the format, so that it does not sag. For laser- or CNC-cut parts where the fit is decisive, the board thickness should be fixed before the design, because it enters the tolerances.

An unsupported thin board veneered on one side tends to cup: the veneer shrinks or swells, the substrate not to the same degree. The solution is either a balancing veneer on the back with the same tension, a sufficient board thickness, or holding it in a frame.

How suitable the substrates are for laser and CNC

Laser cutting and CNC work make specific demands on substrates that go beyond those of veneering alone.

MDF is the most widely used laser material among wood products. It burns evenly, has no grain direction that could deflect the cut, and gives a characteristic dark brown cut edge. This browning can show on intarsia objects if the cut edge is exposed in the finished object. MDF with a low glue content produces less smoke when cut; MDF bonded with formaldehyde needs a powerful extraction.

Birch plywood lasers well, but at the boundaries between plies it sometimes shows different depths of burning, because the layers of glue between the plies burn differently from the wood. The cut edge is lighter with birch than with MDF and looks less dominant in light designs. For visible inner edges in intarsia inlays, birch plywood is visually the better choice.

HDF lasers with a high quality of cut and minimal fraying at the edge. Its higher density needs a little more laser power or a lower cutting speed. HDF cut edges are especially smooth and suit small precise fits in geometric inlays.

For CNC milling: MDF and HDF can be worked with standard woodworking cutters, but their high glue content dulls tools faster. Birch plywood mills cleanly, especially if the tool and feed rate are matched to the number of plies. Details of laser and CNC tools are on the page tools for geometric intarsia.

How Reihax chooses substrates

Reihax has no single standard substrate. Different boards are used for the carcasses of pen holders such as THINK BIGGER: raw particleboard, MDF, plywood or multi-ply. Each of them carries veneer reliably if it meets the requirements on this page.

Raw, uncoated particleboard has long been a usual substrate for veneer in furniture making. Its fine face layers are flat and take glue well; the coarser core only shows at the cut edge, which is therefore covered or veneered. Coated particleboard, on the other hand, is unsuitable, because glue does not hold on the coating.

More decisive than the kind of board is its condition. Every board is checked for flatness before gluing, cleaned of dust and acclimatised in the same room as the veneer. A flat intermediate board in the press spreads the pressure evenly, even if the press jaws have small irregularities. After sanding the flatness is checked again before the finish goes on. How several pieces with carcasses prepared the same way are made in one batch is described in making small objects.