Protecting Back of Wood-Based PanelsApproveRejectUn-ApproveSubscribeUn-Unsubscribe
Question asked 2018-04-11 20:05:35 ...
Most recent comment 2018-04-17 13:39:30
Either pigmented shellac or a solvent-based, alkyd house paint (from the hardware store) were recommended to me as good barrier coatings to apply to the back of wood-based panels to protect them from humidity. A few questions:
- Would acrylic paint work as well as alkyd house paint to seal out moisture? Would it make a difference whether it was an artist grade acrylic paint versus acrylic housepaint?
- Is a solvent-based, alkyd paint recommended because it seals from moisture more thoroughly than acrylic paint? Or does the alkyd not necessarily seal better, it's just more durable?
Answers and Comments
The back of panels is the only place that I really recommend
using shellac on paintings. A good quality alkyd housepaint (I would go with at
least semigloss to ensure that the paint is not too leanly bound nor contain
matting agents that would create a porous film). Alkyd would likely be as good
as or better than Shellac. This is one of the instances where oil or solvent-based
materials win out over water borne media. A film made from a dispersion or
emulsion will always be more porous or permeable than a solvent borne resin or well
bound oil film. Acrylic dispersion are vastly preferable in some situations,
this is just not one of them.
Having written all of the above, we should keep in mind that
the goal of coating the back of a panel is to create a situation where the
front and back of the work respond in a similar manner to changes in relative
humidity. Problems evolve when the surface of a work is impervious to moisture
while the reverse is effected in an unrestrained manner. A work in acrylic
dispersion paints on a panel (executed in 6 layers on 3 coats of an acrylic
dispersion ground, for instance) would be perfectly protected by a similar
stratigraphy on the reverse. If the surface is in oil or if the acrylic
dispersion painting is coated with a solvent borne varnish, it is probably best
to use a solvent borne sealant on the reverse.
As always, other moderators may have a different take on this.
Sorry for the belated response. Kristin and I took an
internet break and left town to celebrate our daughter’s first birthday over
A few thoughts about you earlier post. First, it is preferable
to avoid hanging paintings on the interior side of an exterior wall. This is a
situation where there the front and back of the panel will always see a very
different moisture environment. If it is necessary to hang a work like this,
it is really essential to make sure that the panel is very well sealed and not
worry if it is more so than the front.
When I wrote that, “it is best to treat the front and back
of a painting in a similar manner”, that was an ideal that is difficult to
achieve in practice. I would always ere by making the reverse as well isolated
from moisture suffusion as possible on a panel painting. The situation is compounded
with natural wood substrates. Even ignoring the cut of the wood, the side of
the panel that is more absorbent or that receives a higher RH will swell more
than the other. This can collapse the wood cells on the other side creating a
permanent warp. It is always safer for paint for the warp be convex rather than
concave in terms of the painted side (this is also why if one needs to roll a
painting on canvas, it should only be done with paint side out. Compression is
far more damaging than stretching). In this scenario, unrolling the paint side
out relieves pressure while unrolling a painting rolled paint side in causes
additional stress. A similar situation exists for painting on natural wooden
panels, just to a lesser degree.
This rather answers your last post as well. In practice, it
is probably best to fully seal the back of your panels irrespective of whether
they are varnished or not. It is both difficult to replicate the layering on
the front and it is always possible that varnish could be applied to the surface
in the future (although a conservator should always follow the direction of the
original artist in terms of whether a work is varnished during a conservation
I hope that is more clear.
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