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Wood Panels For Painting


Wood is used as a painting material from long back in history and is still considered one of the best materials for painting.

Lot of labor went into creating big sized panels but now we have many engineered woods which we can use as a painting substrate. Preparing panels for painting in todays day is much easier than it was a few centuries ago, If you read this article it will give you all the necessary information in selecting a proper substrate which will make your paintings last longer and that is what we all want if we are serious about art.





Substrate/Support


What is a substrate?


A substrate is the surface that carries our paintings, it is the structural element of the project, and if it fails the painting surely will not last longer.

So be very careful about the surface you are painting on because it is as important as what you are painting.



Rigid panels


One of the rigid support was wall which was used in earlier times to carve the artwork on them, another one is wood, in this article I'll be only concentrating on wooden substrates easily available in market.





Solid wood Panel



Renaissance painters made panel making an art, now if you haven't read my blog about panel making techniques (used by old masters) , I would suggest you to go to that and read it or watch the video on same to save some time. It will give you brief idea on how woods were used as panels.


Sophisticated joinery, rigid and huge structures and conservation methods used by conservators have saved the paintings done on solid wood panels till date.

But solid wood panels posses a huge problem.


Wood is cellular structure. It contains cell which store water, after cutting the log from the tree the cells loose water in them, but still are capable of absorbing water again resulting in expansion and compression due to changes in atmospheric moisture.

And that is why seasoning is very important. Drying of wood over long periods of time will make the cells stable.

Don't ever use un proper seasoned wood. The artificial seasoning we are using today which is the kiln drying does not produces well seasoned wood.

the wood is cut in two different ways:

Radial cut

Tangential cut

Radial cut wood :the expansion and compression stresses are evenly divided on both the faces of wood, and thus makes it more stable to moisture fluctuations.

whereas in tangentially cut wood the expansion and compression stresses are unevenly present on the two faces which results in shrinking cracking and cupping of wood over time(in the presence of moisture)

In a lumberyard though it is rare to find radial panels as it wastes lot of wood, but you can ask them if they have any.

Also the radially sawn wood must be seasons properly in stable environment.

The woods you can look for are mahogany and white oak.

the panels should be made moisture resistant by applying proper sizing and ground to both the faces and edges. In order to make it more stable add the paint layer which you used to paint the picture, it will divide the stresses equally on all the sides.

Not many thick trees are harvested these days so it will be difficult to find wide panels. The boards can be joined together to make a larger surface but then again they should be glued properly, if not there will be chances of splitting and cracking.

In case if you have braced the panels on the rear there will be greater chances of cracking as these cradles hold the panels way too tightly against the stresses developed in the wood grain




Other than using solid panels there are many other engineered panels prsent today which have eliminated the disadvantages of solid wood panels.

The engineered wood industry measures some of the mechanical and physical characteristics like the density, internal bond etc in theses panels to guide us to use the wood of whatever characteristic we are looking for.



we'll first discuss the 4 characteristics that are necessary to keep in mind for selecting panel boards for painting:


Density: Density is measured by the weight of the panel in pounds/kgs per cubic foot/meters and is very important because the panels with low density tend to be more prone to warping and more difficult to prime.

when density is increased with any other changes it increases stiffness and internal bond strength and most importantly the susceptibility of moisture absorption which results in warping



Internal bond: is the force that takes to pull a material apart in a direction perpendicular to the surface and is expressed in pounds per sq. inch psi

It is important as it tells how strongly the fibers in a panel are bonded,

if you are working on a thin 1/8'' panel you want to make sure you are working with a stiff warp resistant panel with a minimum bond strength of 150psi.



Modulus of rupture:

is a measure of maximum breaking strength of the board measure again in pounds per sq inch psi

this is the ultimate unit of strength in flexure or bending. In plywood the typical measure of this same strength quality is MOE Modulus of Elasticity.




Moisture content: moisture is always present in varying amounts expressed as a percentage of the oven dried weight of the wood. Water exists in wood in two basic ways: 1) Free Water which can occupy the cell cavities and totally saturate the fiber structure of the wood (up to 200% moisture content, much as water saturates available spaces in a sponge) and, 2) Bound Water which has actually created a chemical bond with the cellulose molecules in the cell walls of the wood (an average of 12% moisture content).


free water is not much of a problem as it will come and go.

Whereas bond water leaves under drier conditions and return back under more humid conditions, casing swelling and warping, if there comes excessive warping the lignin (the natural glue that holds the cells together) will ‘set,’ making the

warp permanent.






Let us now discuss some of the most common laminated panels available in the market today, which we should consider in panels for use as painting surface:





Plywood


Thin layers of wood veneer are peeled from the tree but rotating the log against a sharp knife and then arranging these sheets in direction of the grains perpendicular to each other and then united under pressure by a adhesive bonding agent. The minimum layer is three, the more plies the more stable the panel will be. Beware of the plies whose veneers are paper thin, because it will show cracking once you put primer on it, make sure there are no seams in the plywood. Over time thin plywood veneer will open up as artwork is exposed to different levels of humidity.



The best choice will be die boards which is a plie wood where every ply is made up of same wood and is of equal thickness. This type of plywood is exceptionally stable. Look for five ply board of hardwood. The most accessible and least expensive plywood has pine veneer If one side is smooth free of seams plugs and fillers and knots it can be used for painting.

Softwood grains have a tendency to show through thinly applied paint layers, hardwood veneers of birch maple walnut or mahogany are less likely to show this fault.

If you look at the density of plywood it is lower that MDF and HDF with a surface porosity and roughness of raw wood making priming and sealing it challenging. To its advantage, the plywood is exceptionally dimensionally stable due to cross lamination of plies thereby reducing warping from differential expansion and contraction with changes in moisture content.



A small painting may require a panel of thickness 6mm whereas large one should have a thickness of up to 2.5 cm

A thin panel of large size (24inches) needs bracing against warping on the largest side







MDF/HDF boards



These boards are made through dry process that breaks the particles of wood and reconstitutes them into a new panel using heat and pressure and a binder. One of the most common binder used is urea formaldehyde. This binder shows the problem of outgassing. Though nowadays manufactures are producing panels with no emitting and no added urea formaldehyde. So we continue to see more improved panels in high density boards.

These boards have densities from 55lbft2 to 60lb/ft2. The


manufacturers of high density boards have been able to achieve internal bond strength as high as 270 psi, which is much more than that found in MDF boards which is around 155psi.

The dry process leaves a porous surface and there is some fiber raising in HDF's but it is far better than that in MDF's

Artists choosing to paint on MDF/HDF boards should cautiously gesso both the sides of their panels to improve dimensional stability of the panel.


MDF boards have low density as compared to HDF


boards this means MDF panels specially the thin panels can warp more easily than HDF's.

You can use thicker MDF to mitigate warping but moisture absorption will still be an issue for primer integrity and potential mold growth within the panel. Also when priming these panels extensive fiber raising takes place as the panel absorbs moisture, requiring much more priming and sanding. A denser panel in comparison will accept smoother coating of primer reducing the number of gesso la


yers needed to properly prep the panel for smooth painting ground.

Humidified HDF and MDF boards have moisture contents of 5 – 6% and have a linear expansion of .3% to .4% depending on the manufacturer.




Hardboards:




uses wet/dry process method that relies on the natural cellulosic lignin as binders within the wood to cement the fibers together and make the panels solids, No additional additives are necessary in wet/dry process. This method of manufacture


produces a dense one layer subs


titute for solid wood that does not have pronounced grain. The panels are therefore less likely to warp and are resistant to moisture penetration.

two hardboard finishes are available:

one is tempered hardboards and oth


er is non tempered

In Tempered hardboards a small amount of pH neutral oil (1.8 oz. for a 4feetx8feet sheet) is applied with a roll coater at the time board leaves the press. The purpose of this method is to make the board stronger and less prone to warping.




Hardboards come as S1S and S2S.

In S1S only one side is smooth and in s2s both the faces are smooth as they are made by pressing the fibers between two smooth plates.



Hardboards have high density and thus will absorb less moisture and be less prone to warping or fiber raising as in MDF's case. Tempered hardboards exhibits even more resistant to these problems.



Due to the density and naturally bonded fibers,


humidified hardboard will range from 4 – 5% with the maximum absorption generally not exceeding 9% under the most hu


mid conditions in ambient storage.



Hardboards are found in varying thickness but are more readily found in 4mm and 6mm thickness, standard width is 4feet and length can vary 8,10,12feet

Small panels can be used without an auxiliary support but


all panels may warp if painted layer


is on only one side, it exerts uneven tension in large panels and they will bend and deform from their own weight. Bracing will easily diminish this problem.

\



Conclusions


Hardboards : are the most stiffest and are dimensionally more stable than the rest of the panels, but bracing will be required for larger panels in order to avoid bending in future.

Also sizing and priming and colors should be applied on all faces and the edges to reduce stresses caused by expansion and compression.



Plywood: Die boards can be taken under consideration to paint paintings on them, thin veneers should be avoided as they may peel off later and wont be able to take stresses due to dimensional changes. Hardboard veneers should be chosen for painting.


MDF and HDF: HDF boards are better than MDF as they have higher density than them, but swelling will occur when priming is done, so many layers of priming will be required for them. These boards can be sealed and made dimentionally stable but will still remain prominent for mold growth. High Humid locations will show this problem.





Sources:

justpaint.org

The Painter’s Handbook: Revised and Expanded – Mark David Gottsegen, Copyright, 2006






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