Well, I think I've finished my experiments with gluing canvas to foam.
The experiment was to see what held the canvas to the foam the best. The experiments did not determine for me, "what protects the canvas the best", it was only for adhesive strength. And the best measurement for that, to me, was when I pulled the canvas off the foam, what took the most foam with it.
In this experiment, I'd have to honestly say that full strength Glidden Gripper won. Full strength TBII was a very close second. And watered down TBII and Glidden Gripper were both very weak.
In this experiment, I took a piece of foam and put small divots in it, then sanded it lightly. Then I filled the holes on one side with Full strength Glidden Gripper and the other side with Full strength TBII. I scrapped the paint and glue into the holes and only left a light residue on the surface.
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Two days later, after it dried, I put a painted a small area with GG on the GG prepped side and another on the TBII side. Then I used a little watered down GG about 75/25 on an area on the GG side. I then put a little TBII in an area on the TBII side then on the GG side. Then a little watered down TBII on the glue side.
I then pressed strips of canvas on these areas and allowed it to dry for three days.
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When I pulled the canvas off the foam, the piece of canvas that pulled off the most foam was the strip that full strength Gidden Gripper was used on. (Scroll down on this pic, the strip on the bottom right was gripper on gripper and you can see how much pink it pulled off.)
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As I said, this was not an experiment for seeing what would protect the canvas over long exposure times, it was just about adhesiveness.
However, from the experiments with the 50/50 TBII, it did saturate the canvas fairly well. For my build, I will probably attach the preshrunk canvas with Glidden Gripper then after it has dried, use 50/50 TBII to get into the pores of the fabric, then coat again with Gripper, then paint it.
Dave