I have a question about the use of GAC 100 to size a canvas. This is my first time stretching my own canvases. I decided to size my canvases with GAC 100 to minimize support induced discoloration as I intend that my painting is going to be done entirely in glazes. I thought I followed the instructions for applying GAC 100 but it seems that after two coats of it my canvases have a bit less tension in them than they did before I applied it. It this to be expected of GAC 100? Or does this indicate that I may have applied it incorrectly in some way?
Priming is always something that takes some practice, so keep at it. Like many products, GAC 100 layers are going to shrink upon drying, and sometimes they will loosen the staples/tacks if they have not been set very tight. This can also happen if you stretch the unprimed canvas too taught. The more you prime, the more you'll discover what is too taught. I've had people who have actually bowed their stretcher bars as these coats dried.

If you think the canvas is too saggy, you may want to restretch it. There are some stretcher bar companies that make "keys" for their bars, but these can be very costly.