venetians schools 6-9 coats of size/primer for linen canvas

I will respond in greater detail later but first want to state that I find this number of size and gesso layers very unlikely. 6-7 layers of gesso covering 2-3 layers of size is sufficient for a wooden panel, as long as there is not going to be heavily punched water gilding. Such a large amount of gesso would be extremely brittle on a fabric support. Where did you read about this?

​I will wait until you reply with references before responding. Thanks.

Hi Hector
Technical art history, the study of historical manuscripts (codicology), and the scientific examination of paintings has progressed by leap and bounds since Mayer wrote his tome. First, I need to state that ground preparation was more diverse than though and there could certainly be outliers. Additionally, this heterogeneity increases when comparing one reason to another and especially in the centuries after the 1400s. Maartje Stols-Wotlox is one of the leaving experts in the world on European grounds for paintings. One can also consult cross-section by others and other technical studies. I do not want to appear to be an expert on grounds used in Venetian Renaissance painting; but the following is my synopsis of what I have read from truly informed sources:
One or more layers of sizing, usually, animal glue, although starch appears to have been periodically employed.
A very thin gesso ground (again starch bound gypsum is a possibility for some). Often this ground appears to have been scraped into the interstices of the fabric. No matter what this needed to be thin as any real thickness would have been very brittle and likely to crack and delaminate.
This was very often covered by another layer (often termed imprimatura, although today many think of this layer as being very transparent, that is not always the case), often composed of pigments in oil but sometimes clear oil. The oil containing layer served to cut the absorbency, facilitation paint application. It also could suffuse the brittle lower ground providing some flexibility.
When red or dark ground come into fashion (Tintoretto and El Greco are early users of such grounds) they were  often made by sizing and then scraping on a thin gesso ground. This was then covered with an oil containing priming, often composed of palette scrapings.
As you progress into the mid and later 16th century there is more heterogeneity. Some use the double ground system while others abandon the lower, water-soluble ground for just oil grounds. Grounds becomes even more varied as the decades and centuries progress.
The above is a massive oversimplification and I am sure that Maartje and others could nitpick some of my over generations. However, it is essentially what we believe about grounds from the period you mention. Additionally, while I cannot give you the exact number ground layers that a specific Venetian Renaissance art would use, it is highly unlikely to be the number suggested by your readings.  

​Sorry, I just read you last post, that number does make far more sense for panel, although it may of may not be sufficient for complicate gilding techniques.