PVA for sizing

PVA is available in a number of forms as consumer products and art supplies, so I think it would be too broad to say this material is overall unreliable. Not all PVA products are equally durable; the ones sold as artists’ sizings and archival adhesives are made for permanent applications on objects intended to survive into antique age. Scholastic-grade glues and stationery products generally don’t carry any assurances of permanence.

Personally, I have always gotten the best results using acrylic-based sizings. There are some which even rival RSG for stiffness. ​

The most damage occurs when there are rapid and wide fluctuations RH, more so than very slow changes. That is true unless that RH gets above 85% when the RSG has no rigidity at all contributes no support. The canvas is then free to shrink and the paint can do little but crack or delaminate. This is far more dangerous with older paintings where the oil paint has become very brittle over time.
I am with Matthew on PVA vs acrylic dispersion sizes. I see no reason to choose PVA over the later. 

​I have forwarded your question to a couple of relevant moderators

​ ​All organic coatings will yellow to some degree. High quality acrylic dispersion mediums formulated for the fine arts yellow to only the slightest degree. The problem is when the gel is applied in VERY thick layers, even the tiniest of yellowing becomes visible.

Hi Kimberly, yes, acrylics as all organic polymers can yellow, but the industry is competitive and paint manufacturers are continuously improving their formulations. Thus good quality acrylics mediums today might be significantly more resistant to yellowing than the product you used 15 years ago. Furthermore, yellowing does not necessarily mean that the mechanical stability of the coating is compromised. ​​

The problem with this line of questioning is a discussing PVA as if it were one material.  If fact it is a class of materials.  When CCI did their long study of PVA and acrylic adhesives ( https://www.iiconservation.org/node/877) they included 52 different types of formulations, 
e.g. different types of homo-polymers and co-polymers. The study details that the different formulations gave very different results in terms of acidity, flexibility, and color change.
Some types of PVA formulations performed very well, other types not so well, the same is true for acrylic adhesives.  
 
I cannot say what any other company is doing. But speaking for Gamblin Artists Colors, in formulating our one product based on PVA, our PVA Sizing,  we chose a type of PVA identified by the CCI study to have low acidity, retain flexibility, and to have low color change.  We then did the testing to determine what concentration of that adhesive would do the work that we have come to expect of rabbit skin glue when used as a sizing. That is, to protect canvas and paper from the penetration by oil into the fibers. 
 
In terms of the advice I can give in this area, I would not assume that a PVA adhesive on the market formulated to bond wood, paper, or ceramics would necessarily have the properties of stability that we are looking for to make permanent paintings.   These products may perform in a way we would hope, but they also may not.  
 
I hope this is helpful, 
Robert Gamblin

​Sorry, my last post was truncated due to server issues. Here is what I was trying to write: ​ ​All organic coatings will yellow to some degree. High quality acrylic dispersion mediums formulated for the fine arts yellow to only the slightest degree. The problem is when the gel is applied in VERY thick layers, even the tiniest of yellowing becomes visible.