Polyester canvas for oil paint

Seeing the evocation of “Dare to Be First” staring at me at the bottom of the Forum page, let me take up that challenge while waitting to hear Brian’s thoughts as well.
Given that cotton canvas, or really any textile, is already far more flexible than any oil paint film, especially as those films age, I think there are potential, positive tradeoffs in terms of the polyester canvas being less reactive to environmental changes. Also, like with canvas, the ultimate stiffness and flexibility of the support – and really the materials that will serve as the carrier of the painting overall – will be any sizing and ground that you use.  In your case we would think that initial coats of acrylic gesso followed by 1-2 coats of an oil ground, should provide a stiffness likely not far off from a similar treatment on  something like portrait linen. At the same time I have not looked deeply into what are the longer terms issues – if any – with polyester canvas experiencing creep or loss of strength. And one would still be confronted with canvas of any form never being the ideal substrate, so some risk will continue to be locked in regardless.On a final note, the late Ross Merrill, past Chief Conservator at the National Gallery of Art, was fond of using Sunbrella fabric as a support for his own paintings, which is a more substantial acrylic textile most commonly seen used for exterior awnings. See the following as one point of reference:
http://cool.conservation-us.org/byform/mailing-lists/cdl/1999/0522.html
So another option to look at that might be inerently less flexible and more durable, even, than polyester. but again, we have no direct testing of our own to go on.
Sarah Sands
Senior Technical Specialist
Golden Artist Colors

Kevin and Sarah
Sarah makes some very valuable points and I have no disagreement with them at all. She is spot-on when pointing out the benefit of removing the effect of changes in environment on the substrate. I do not want to make too big of a deal on this, but I do have some concerns about the pre-primed polyester canvases that I have examined.
I will try to clarify my general feelings about polyester, especially very thin pre-primed fabrics, as a substrate for oil paint, and especially oil paint that is applied in any degree of impasto. Yes, the size and ground contribute much of the required stiffness for the painting. If we take this to the extreme, the same could be said of applying size and ground onto onionskin paper. A heavy application of size and ground would make this system “suitable” for oil painting. At a certain point, I begin to ask what the support brings to the table? Would it not be better to just have a thick slab of acrylic dispersion ground and forgo the overly flexible fabric all together? Yes, the polyester fabric does provide some “very minimal” texture but could this not be added to the surface of the acrylic substrate? I am intentionally exaggerating here to make a point, but I do think that it is a point worth considering. I also have similar concerns about all thin, flexible fabric supports and especially the very insubstantial, thin pre-primed fabrics ubiquitously sold in craft and fine art stores alike. 
As to the available pre-primed polyester fabrics. Those that I have examined (and I likely have not seen them all) have been extremely thin and very flexible, almost elastic. To me, this does not equal the required degree of stiffness of support to make them suitable for oil painting of any appreciable thickness.
Back to the Kevin’s original plan. As it is written, I see a very flexible substrate, if the canvas you are speaking of is in any way similar to what I have examined, with a layer of oil paint applied to it (in this system, the lead white ground will simply function as a preliminary application of quite lean oil paint). No doubt, lead white is very flexible but this gives me pause. On your finished painting the lead white ground and all subsequent oil layers with become more brittle with age and will be sitting on a potentially extremely flexible support (again, if the polyester canvas is similar to others that I have seen). I am not sure if we can say that the polyester’s lack of response to environmental changes justifies this situation. I personally err on the cautious side of thi for the moment.
Now I do not want to tell you that you should not follow your proposed plan. It may turn out that the system you mention will work perfectly fine for your work. I simply have the above reservations. Additionally, I do want to give the impression that I am extolling the virtues of linen and cotton in general. The use of a heavy polyester canvas with less inherent flexibility (or one with a substantial size and ground layer, which provides a good degree of stiffness). This may really be a profound improvement for those who want to continue to use flexible, lightweight supports but want to diminish the effects of changes in humidity on the preservations of their oil paintings.  
So those are my thoughts. I really welcome additional dialog on this issue. I am also very happy to change my mind on this if the research indicates it.