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Question asked 2018-12-27 21:19:28 ...
Most recent comment 2019-01-10 18:22:41
Grounds / Priming
I am currently working on birch plywood cradled panels that I have first sized on both sides with 2 coats of GAC 100, then applied thin layers of traditional glue gesso for painting in casein and oil. In a couple of the paintings I have seen a few areas of short parallel cracks coming through, and am considering adding a layer of thin fabric to the preparation process. Before I get in too deep with preparing a bunch of these panels, I'll appreciate some advice on 1.) the best material or materials to seal front and back of panel, and is the GAC 100 OK for this; 2.) best adhesive(s) for attaching fabric (I've used acrylic gloss medium for this in the past, but have had hide glue suggested to me as better).
OR -is there a way to avoid the fabric step altogether, and simply prepare the wood surface well enough to accept the gesso? It seems like I've read various opinions on this and am now pretty confused! Thank you for your help.
Answers and Comments
If you read through our “resources section” and do some relevant
searches, you will see that we have discussed most of these issues. Even so,
here are a few thoughts: I do not recommend plywood panels for traditional
gesso, chalk glue grounds, or acrylic dispersion grounds without some sort of
interleaf, preferably of fabric. For 25 years I have seen the surface of panels
created by very good craftspeople exhibit checking along the grain with all of
these grounds (do a search here to find more discussion of this).
Additionally, I see no reason to use an acrylic dispersion
medium under an animal glue/chalk or calcium sulfide ground. The ground negates
any of the stability benefits of the size and creates a situation where the
more brittle layer is over the more flexible one. I would size with animal glue
if intending on using an animal glue bound ground.
Please peruse the site but also feel free to ask any
additional query on that, or any other, subject if you feel that we have not
covered your particular question.
Just wanted to add some notes as our GAC 100 was referenced. First, we would not recommend any acrylic medium under a traditional gesso, hide glue/ chalk ground. The adhesion is just not ideal as those grounds really need a very absorbent surface to lock into. So would definitely follow Brian's recommendations on that.
Second, just to clear up the role of GAC 100 as a "sealer" as it is being too often misunderstood. GAC 100 will NOT 'seal' a panel off from moisture or humidity, which are really the things you want to control. In fact acrylic coatings score quite low in terms of providing an effective barrier to moisture. However, GAC 100 or Gloss Medium are used to block something called Support Induced Discoloration, but that is only a concern for acrylic paints working in specific ways. In your case, having GAC 100 on the sides and back would at most provide a physical barrier, but being as soft as they are, they will also easily become dirty. Instead, if wanting to truly seal a panel from changes in humidity, we would recommend using 2 coats of an exterior alkyd-based wood primer, and if wanting the ultimate moisture seal, follow that with a paint made with aluminum flake. Most major manufacturers such as Sherwin-Williams®, Benjamin Moore®, PPG®, and Rust-oleum® produce these, where they are commonly used for marine, metal, or automotive applications.
For an excellent list of the coatings with the best ratings for moisture-excluding effectiveness (MEE), we would recommend looking at the table on pages 13-15 in Williams, R. Sam's “Chapter 16: Finishing of Wood”, Wood Handbook, Madison, WI, Forrest Products Laboratory:
As always hope that helps clarify things.
As oil grounds are, at least initially, more flexible than
glue grounds they probably fare better…..at least in the short term. Again,
however, I really do not recommend plywood substrates for oil painting grounds without
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