MITRA Forum Question Details

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.

Koo

Sorry for the belated response. Kristin and I took an internet break and left town to celebrate our daughter’s first birthday over the weekend.

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 treatment.

I hope that is more clear.