Sealing wood panel for acrylic paint: Shellac BIN or GAC 100? Warping?ApproveRejectUn-ApproveSubscribeUn-Unsubscribe
Question asked 2017-04-20 12:46:55 ...
Most recent comment 2017-05-01 14:21:42
Grounds / Priming
Art Conservation Topics
Debating whether to use pigmented Shellac BIN or GAC 100 as sealer. From what I've been told, Shellac BIN is a sealer, GAC 100 is not. (regular Shellac can dissolve acrylic paint due to alkaline sensitivity to ammonia but Shellac BIN seems to be ok)
GAC 100 reduces SID, but so can Shellac BIN.
It seems like Shellac BIN is winning out here... I plan to put a couple layers of Gesso on top of either Shellac BIN or GAC 100 before painting of course. If Shellac can do what GAC 100 can do but it is a true sealer wouldn't Shellac BIN be a better choice?
Regarding WARPING: Someone told me that shellac also can prevent warping due to blocking moisture.. is this true?
I'd like to eventually work bigger than 48" at some point and use the thinnest plywood possible (prob birch) to keep it light and of course cradle and brace it with supports. But what are your thoughts as to warping at this size? Would getting 1/4" be too thin? What if I put 3 layers of Shellac BIN?
Answers and Comments
So first to address the notion of a "sealer"....I guess what you mean by GAC 100 not really being a sealer is that it will produce a slightly porous film. Additionally, being water-based GAC 100 may contribute to the warping of thin panel substrates. If I were leaning towards using a water-borne size, I would likely go with a couple of coats of GAC 400 applied to both the front and the back (I believe this is detailed in full on Golden's website). It is also true that a solvent-born size (such as this particular shellac primer you mention) is far less likely to cause warping of the panel. Resins dissolved in solvent tend to create a more continuous, non-porous film than do dispersions such as GAC 100. However, in practice this is probably not a huge issue as you will ultimately be applying an acrylic ground atop whatever size you choose. In general it is best to treat the front and back of your panel in a similar way in order to avoid short-term warping.If I were interested in looking into a non-aqueous sizing material to use on a panel I would explore using solvent-borne acrylics such as Paraloid B-72 dissolved directly into ethanol/acetone and apply in one or two layers. The reason for this is that even bleached Shellac does show less than desirable long-term aging properties, despite some opinions to the contrary. Again, in practice this is probably less of an issue if the shellac is used as a size as compared to a surface coating.
We are slightl confused about the mention of ammonia...the last time we checked (albeit some time ago) the BIN was a pigmented shellac dissolved in ethanol. Perhaps we are not up to speed on this...I tend to think of ammonia being present in dispersions and emulsions.
As for the size/thickness of your panel....the thicker the better of course but you will absolutely need some form of bracing on the back. Please refer to our "Rigid Supports" document in the Resources section for additional info on panels and possibly the "Adhesives and Sizes" document as well.
Just a quick correction on something you said:
"...regular Shellac can dissolve acrylic paint due to alkaline sensitivity to ammonia but Shellac BIN seems to be ok"Actually, as the following articles point out, it is actually the reverse that is true - with some acrylic products dissolving shellac-based ones due to shellac's alkaline sensitivity:With acrylics, there is some alchohol sensitivity, so putting BIN on top of acrylics would likely bite into them some. But not because of any ammonia sensitivity - that you have a bit backwards.
Hope that helps!
Senior Technical Specialist
Golden Artist Colors
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