Trying one more time with the PVC
Yesterday afternoon, I spent some time researching solvents for PVC. Kaszuba gives an ascorbic acid subbing formula for PVC in the patent, above, mainly including a lot more acetone and less methanol, but I could not get it to work. Ian makes a good point that there might be some other coating or ingredient from the manufacturer. Grafix doesn't tell you what's in it in the MSDS.
Turns out PVC is pretty tough stuff but the main solvents are acetone, MEK and tetrahydrofuran. Acetone and MEK are weak solvents; the best, tetrahydrofuran. These solvents are also the ingredients of common PVC pipe glue along with a small amount of PVC resin. So, I tried adding a bit of PVC glue to the subbing formula. Unfortunately, this precipitates the gelatin in the subbing into a hard ball. Next attempt was to mix a solution of the glue with acetone and apply that mix to soften the PVC. If I apply this first dilute glue solution and let it dry, then apply the regular subbing, and then coat gelatin, the gelatin sticks very well and does not pull off the PVC.
The glue mix can do some nasty things to the PVC support so it appears that a very dilute solution is needed, since we just want to soften and not dissolve. The correct mix will likely be somewhere between 50:1 and 100:1 acetone to glue. The last attempt I made last evening was 30:1 and the PVC base is too optically distorted to be able to print from it. But the gelatin remained stuck firmly to the base.