A Wood Panel Primer:
Basic Wood Science For Panel Painters

What every artist should know about wooden art panels, but was never taught in art school

Cellulose	- cellulose is the main constituent of all plant tissues. In wood, it takes the form of long, thin, tubular fibers. These fibers make up the basic mass and structural material of wood, and function as conduits to passively transfer moisture, nutrients, and waste gases in and out of the wood. It forms both the structure and vascular system of trees. It is a remarkably strong, durable, and long lasting material. Cellulose fibers bond with each other end-to-end, and are also bound together side-to-side by lignin, woods own polymer "glue". Lignin also hardens cellulose, and gives it its rigidity. When it is "fiberized", or shredded, the wood fibers can be made into fiberboard. Those same fibers stripped of their lignin stiffener can be made into paper. - (See also: lignin and deterioration)
Delamination	- The separation of some of the layers that make up a painting is known as delamination. Delamination of the panel itself is most easily recognized in plywood, where the multiple layers of wood veneers can separate from each other. Since the individual veneer layers are very fragile, this can have a disasterous outcome. Hardboard (Masonite®) also has a tendency to delaminate; the structure of the wood fibers in hardboard is planar in orientation, and the fibers tend to lose adhesion in layers. To make hardboards smoother, they are manufactured with a surface layer of very fine short fibers called "fines". Many brands of hardboard have a substantial layer of these "fines", which can – and often do – pull away from the main body of fibers very easily. (See also "wet-process".) Delamination can also refer to the separation of paint from a board or canvas, and to the separation of different layers of paint, as when artists paints separate from the primer. In panels, delamination can be minimized by using stronger adhesives and binders. Fiberboards whose fibers are entwined in three dimensions don't face the delamination problems of hardboard, whose fibers align in two dimensions (see "dry-process boards"). Between paint layers, adhesion can be maximized by a micro-textural grip, by chemical bonding between layers, and by strong adhesive properties in the paint and its curing method (See our "Advanced Primers" page).
Deterioration	- Most deterioration of wood is caused by the breakdown or loss of lignin. Lignin, a naturally occurring polymer and a major component of wood, is the glue which holds wood cellulose fibers together. (The listing for lignin contains a more detailed look at lignin loss and deterioration.) Live wood constantly replenishes lost or damaged lignin in order to maintain its strength. When the wood dies, its supply of lignin becomes finite. For that reason, securing that supply of lignin is crucial to prolonging the life-span of the wood. Deterioration caused by direct destruction to the more durable cellulose components of the wood is less frequent. When this occurs, it is usually caused by vermin, bacteria, or fungi. - (See also: lignin, sealing panels.) (Also see our "Deterioration of Wood Panels" page.)
Dry-process boards	- The term "dry-process" refers to one of the two methods used to make wooden fiberboards. (The other is the "wet-process".) Dry wood fibers are mixed with strong and stable synthetic resins. Piles, or "mats", of this mixture, usually several feet thick, are placed in massive presses. The mats are compressed under high heat and pressure. The resins melt, binding the fibers together into hard, dense panels. The fibers in the panels are tightly interlocked and oriented randomly in three dimensions. The panels are prized for their internal bond strength, dimensional stability, and resistance to warping, as well as their long-term durability. All MDF and HDF production utilizes the "dry-process", as does a small percentage of hardboard production. (View our Fiberboard Comparison Chart .) - (See also: fiberboard, MDF, and HDF.)
Fiberboard	- Any of several types of engineered wood panels made from "fiberized" wood. The wood is shredded and treated in order to separate it into individual fibers. The fibers, with or without the addition of polymer resins, are formed into boards using heat and pressure. Industry standards group fiberboards by density into three categories: Hardboard, HDF, and MDF, although fiberboard manufacturers often blur, stretch, or interchange these terms (see the individual listings for each). A useful distinction can be made between "wet-process" boards (most hardboards) and "dry-process" boards (HDF and MDF). The fibers in wet-process boards are "glued" back together using the relatively unstable lignin resins found in the wood; dry-process boards substitute stable synthetic resins to bond the fibers together. The fibers in wet-process boards are interlocked in two dimensions, at best. These boards tend to separate in layers, distort when the fibers expand, and have very porous edges. Dry-process boards are made with fibers interlocked in three dimensions. This makes for a more cohesive board which minimizes de-lamination and edge porosity, and increases dimensional stability. The two types of fiber structures can be compared to those found in machine-made and hand-made papers. Dry-process fiberboards are very stable and make excellent art panels; choose them over wet-process boards for this purpose. Fiberboard should not be confused with cheaper particleboard. (View our Fiberboard Comparison Chart .) - (See: wet-process, dry-process) (See also: hardboard, HDF, and MDF)
Gesso Panels	- Traditional gesso is a wood primer made of hide glue (usually rabbit skin glue), fish glue, or casein, and is traditionally mixed with calcium pigments (chalk, whiting, marble dust, etc.). It is water miscible (slightly re-soluble) and very absorbent. It was designed (and is now properly used only) for egg tempera and encaustic paints, being too absorbent for other media. It is delicate, but it can be durable if carefully prepared, applied, coated, and maintained. So-called "acrylic gesso" and "synthetic gesso" are made from acrylic or acrylic/PVA latex emulsions. These are used under acrylic paints and – probably all too often – under oil paints as well. They are permeable, but have relatively low absorbency. - (See: primers) (Also see our "Advanced Primers" page.)
Hardboard	- (Often referred to by the trade name Masonite^®.) A class of fiberboard, usually distinguished by the very high temperatures at which the boards are formed, and the extreme density of the finished boards. They are formed at a temperature which is high enough to re-polymerize the lignin ("nature's glue") in the wood; the lignin serves as the binder which holds the board together. (Some hardboard has other binder resins added in addition to the lignin.) Hardboard may be either "tempered" or "standard" [untempered]. All tempered hardboard made today is treated with a thin coating of linseed oil (at one time tung oil and other drying oils were used) to increase strength, water-resistance, and paint hold-out. Tempered hardboard is a poor choice for making permanent artist's panels, especially if they will have acrylic or other water-based primers or paints applied to them, as the long-term adhesion of these coatings to the linseed oil is questionable. Films of straight linseed oil, especially the boiled or blown types used for tempering, have poor durability and make an unstable base for paintings. It is easier and cheaper to get a smooth coating of paint on a tempered board – hence their use in inexpensive commercial art panels. The tempering renders them largely non-absorbant, and also reduces the raising of fibers by liquid paints. This greatly reduces the need for sanding, but also greatly compromises the adhesion of sealants or primers. Adhesion of paint is somewhat better on untempered hardboard. Although hardboard, and especially the tempered variety, is a physically strong panel, it may not be the best first-choice for making art panels. Lignin is a much less stable and durable binder than the resins used in other types of fiberboard, making hardboard the more prone to deterioration. Also, the extreme density and two dimensional fiber structure of hardboard makes it more prone to warping when the fibers expand (they have no room to give!) as compared to other types of fiberboard of lower density. Hardboard is also prone to delamination. For home-made art panels, the best hardboard is untempered "dry-process" hardboard, which is more accurately an ultra-dense HDF. Unfortunately, relatively little dry-process hardboard is made, and it is hard to find . In museum collections, works on hardboard are displaying a varied bill of health. Some are holding up nicely, while others are deteriorating badly. This is probably due to the many different types of hardboard available, and the different ways in which they were prepared. (View our Fiberboard Comparison Chart .) - (See also: wet-process, Masonite^®.)
HDF	- High Density Fiberboard (HDF) is a dry-process type fiberboard panel with a density which falls between that of MDF and hardboard. This also gives it properties of strength and warp-resistance which fall between those two products. HDF is made with the same dry-process techniques as MDF. Dry wood fibers are mixed with polymer resin and then are bonded together using heat and high pressure. (Most hardboard is made from a slurry of fibers which are pressed together in a wet state, at higher temperatures.) HDF and MDF are considered the most stable of engineered wood panels. Laminated with fine wood veneers, they are often used in high quality furniture making and are prized for their warp-resistance and durability. They make excellent art panels. PRIMEWOOD™ panels are made of HDF. Terminology can get confusing: thin HDF (THDF) of 1/4" thickness and below is usually referred to as "thin MDF" or "TMDF" since more people are familiar with the term "MDF" than "HDF". (View our Fiberboard Comparison Chart .) - (See: fiberboard and MDF.)
Lignin	- Lignin is a natural component of wood. After the cellulose fibers, it is the second most abundant material in the wood. A naturally occurring organic polymer, lignin is often called "nature's glue". By stiffening the cellulose fibers, it is what makes wood "woody" and gives it rigidity. It also binds the fibers together. Unfortunately, it is an unstable and impermanent polymer. It is easily broken down by oxygen and UV radiation. It is slowly volatile, and under normal circumstances it will gradually volatilize off from the wood, escaping along with moisture as the wood "breathes" in and out. It is actually the loss of lignin – and not moisture – which causes wood to "dry out" and deteriorate. When enough lignin is lost, the wood can split and crack, decay, and eventually fall to dust. Prolonging the life of wooden objects requires sealing them tightly, thoroughly, and opaquely. This protects the lignin from the effects of light and air, and keeps it from escaping. Most hardboard relies almost exclusively upon lignin to hold itself together (more so even than raw wood, in which there is some direct fiber-to-fiber bonding). MDF and HDF panels substitute more stable and durable polymers to do the job of the lignin. - (See also: wet-process.) (Also see our "Deterioration of Wood Panels" page.)
Masonite^®	- Masonite^® is a registered trademark of the Masonite Corporation. The term is often and incorrectly used to refer to any brand of hardboard, which was invented by William H. Mason in 1924. The Masonite Company, which he founded, uses the name Duron^® to refer to its current hardboard products (which are "wet-process" boards). - (See also: hardboard.)
MDF	- Medium Density Fiberboard (MDF) is a dry-process type fiberboard panel with a density below that of HDF and hardboard. This lower density makes MDF the most warp-resistant wood panel available. Unlike the very dense hardboard fiberboards, fibers in MDF panels have the necessary room within the internal structure of the board to expand (from moisture or heat) without distorting the panel. While this lower density gives MDF somewhat less structural strength than other fiber boards, it is still very hard and strong, and more than adequate in that regard for use as an artist panel, for which it is very well suited. It is much stronger than particleboard, a product with which it should not be confused. MDF and HDF are very similar panels and share most of the same qualities; they tend to differ only by a small amount in density. The manufacturing process of these products usually results in the thinner panels having slightly higher densities. Panels of a half inch or more are generally MDF; panels of a quarter inch or less usually fall into the category of HDF, though these are generally called "thin MDF" (TMDF) since the public is more familiar with the term "MDF' than "HDF'. (View our Fiberboard Comparison Chart .) - (See: fiberboard and HDF.)
MDO	- Medium Density Overlay (MDO) board is a wood panel often used in the sign making and cement forming industries. It is usually made of plywood, coated with outer layers of resin-impregnated kraft paper. Phenolic, urea, or melamine resins are the common resins used on the paper overlays. Because of the impermanent nature of the kraft papers used, MDO is not considered a suitable material for use in permanent artworks. It has been suggested that a home-made museum quality MDO board could be produced by laminating plywood with urethane soaked rag papers. Let us know if you try!
OSB	- Oriented Strand Board (OSB), (and it's obsolete cousin, Wafer Board, which is "un"-oriented strand board) is a strong, engineered wood panel used extensively in the construction industry. It is made of "strands" of wood, thin slices usually 1 - 2" wide and 3 - 6" long. These strands are laid down in layers, often in alternating directions (thus, "oriented"), and then glued together under heat and pressure using strong and durable polymers. The resulting panels possess great structural strength and a high degree of warp-resistance. In construction, they are often used to replace plywood panels. Compared to plywood, they are cheaper and tend to lay flatter, though they also break more easily. In theory, they could be used to make acceptable artist panels. Their surfaces are usually quite irregular, and thus they are hard to coat smoothly and evenly with paint. - (See: sealing panels.)
Particleboard	- Particleboard, sometimes called "chipboard" (Flakeboard^® is the company name of an early maker of particleboard) is a panel made of wood that has been chipped into small splinter-sized bits, and then glued back together using synthetic resin polymers in much the same manner as OSB. It is inexpensive, and resistant to warpage. It is commonly used to make low priced furniture, cabinets, and shelving. It fractures and crumbles easily. Its delicacy in that regard makes it largely unsuitable for use in artist panels. It should not be confused with the much higher quality fiberboard.
Plywood	- Wooden panels made by cross laminating multiple layers of thin wood veneers. This construction makes them stronger than solid wooden boards, as the grain of the veneers run at right angles to each other. This gives them greater dimensional stability, less likelihood of warping, and less chance of splitting through (although individual veneers, especially the outer layers, still tend to split with age.) Hardwood and softwood versions are available, although most "hardwood plywood" is actually made of softwood cores with thin hardwood veneers used only for the outer "skins". Though some species and grades of veneers take paint better than others, it generally requires a great deal of work to achieve a smooth paint finish on plywood without the wood grain showing through. Even when this is achieved initially, the grain tends to become more pronounced as the panel ages. To make truly long-lasting plywood art panels, it is essential that they be properly prepared. - (See: sealing panels.)
Primers	- The main functions of a wood primer are to grip your paint and provide your colors with a strong and secure anchorage to the panel. Sometimes the primer also serves as a sealant. Three processes can come into play in primer bonding: absorption, chemical bonding, and physical bonding. Traditional hide-glue gesso , for example, – which is often given a very slick finish and so provides little physical bonding – relies mostly on absorption to hold colors. (This is problematic for oil colors – and for factory-made paints in general which are not extended with mediums and extra binder – since the loss of part of the binder to absorption can make your paints brittle.) Freshly home-applied oil primers can cross-polymerize with oil colors during curing; they offer an example of good chemical bonding. (Never apply water-based paints over oil primers.) Most primers however, rely primarily on physical bonding ("tooth") to grip artist paints. Before the advent of hardened-steel pigment mills, the particles of pigment and filler in artists' paints and primers were relatively large, and they provided the necessary tooth for a strong grip. Steel mills changed that; pigment particles are now made much smaller. In formulating primers today, unless careful attention is paid to including large particles in the coating – which it rarely is – bonding quality can suffer. Many painters complain that their acrylic primers feel too slick or slippery for just this reason; large particles ensure controllable paint handling as well as a durable bond. (Note as well that the wood itself must possess this same quality [sufficient micro-texture] in order to bond securely with the primer! Beware of using tempered hardboards and glossy seal coatings, or very absorbent wood, or your primer will not adhere tightly to the wood.) (Click this link for more about particles and micro-texture.) Primers serve to protect wood against damage caused by UV radiation. In general, artists' primers DO NOT provide the tight vapor barriers needed to protect wood from the damaging effects of moisture and vapor exchange. (One exception is Primewood™.) See the entry for sealing panels for more on this subject. Wood primers can be made from animal glues or casein ("true" gesso), linseed oil, acrylic or PVA latex emulsions (called "acrylic [or synthetic] gesso"), shellac, alkyd resins, and urethane resins (Primewood™). - (See: sealing panels and gesso) (Please also see our "Advanced Primers" page for more on this subject.)
Sandwich Panels	- Engineered multi-ply panels, usually employed to provide strength and light weight. Whereas plywood is usually made of 5 or more layers of similar material, sandwich panels are usually made of 2 thin skins bonded to one thicker, lightweight core. Many use cores made of honeycomb material. Foam is also a common core in sandwich panels. Foamcore boards are one familiar type. Another type of core material – used in our PRIMEWOOD™ PLUS panels – is end-grain balsa. Balsa wood timbers are sliced like bread; then the slices are laid edge-to-edge to form sheets. In this configuration, the wood fibers align to form a natural "micro-honeycomb" structure. Panels made with these balsa cores can be sealed very effectively, since none of the end grain of the wood is exposed on the edges of the panel. Sandwich panels use the same engineering principle as I-beams to achieve great rigidity: when the panels faces are moved further away from the center line of the panel, they gain progressively more "leverage" to resist bending. In general, sandwich panels possess the highest strength-to-weight ratio of all panel types.
Sealing panels	- To make truly long-lasting, archival wooden art panels, it is essential that they be properly and carefully prepared. This necessitates sealing them meticulously with the tightest possible vapor barrier sealant on all surfaces (we use and recommend urethanes) in order to minimize vapor exchange and loss of lignin, and maximize dimensional stability. (Care should be taken that the sealant not be too glossy, or it will provide inadequate adhesion for the primer!) Most importantly, the edges of the panel must be completely sealed (caulking is one method) in order to minimize moisture and vapor exchange. Panels should then be primed on all surfaces to protect against UV damage. Finally, all surfaces – not just the face of the painting – should eventually be varnished. Sealing a wood panel also helps to protect its cellulose fibers from fungus and bacterial decay. - (See: lignin and deterioration.) (Also see our "Deterioration of Wood Panels" page.)
Wet-process boards	- The term "wet-process" refers to one of the two methods used to make wooden fiberboards. (The other is the "dry-process".) Fiberized wood and water are made into a slurry. The slurry is formed into sheets on belts or screens, much as in the process of paper-making. After some of the water is removed, the wet sheets are put into presses where, under extreme heat and pressure, the remainder of the water is removed and the fibers are compressed into boards. Wet-process boards are made under much higher heats and pressures than dry-process boards. The higher pressures result in higher density panels. The higher temperatures are extreme enough to re-polymerize the lignin resins found in the wood. It is the woods own lignin resins which bond the fibers of wet-process boards together. Some wet-process machinery extrudes the wet slurry onto moving mesh belts. This results in panels in which the fibers are structured in parallel alignment with each other ("grain"), akin to machine-made paper. These panels are the most susceptible to warpage. Better grade wet-process boards are made without moving belts, and the fibers are randomized in two dimensions, as in mould-made paper. A variation of the wet process called "wet-dry process" also uses a wet slurry, and also depends on lignin as the binder. "Wet-process" boards are smooth on one side, textured on the other; "Wet-dry" boards are smooth on both sides. Most hardboard is made by the wet process. (View our Fiberboard Comparison Chart .) - (See also: fiberboard, hardboard and lignin.)