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Originally Posted by Thomasin Dewhurst
... except it's "medicinal purposes" perhaps? . . .
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Ha ha ha ha. Thomasin, I started getting some "strange" e-mails after purchasing hemp canvas from Pickering . . . care to join the "Legalize Marijuana" movement?
Putting full-strength RSG on the back of a completed canvas is not the best idea, nor is it the best way to correct a canvas that's slacking off. On the other hand, it's not a huge error that will result in the piece self-destructing before your eyes, and the only way to do that successfully would be to apply it with the canvas attached to the chassis.
Approaches for correcting tension problems in a finished canvas, depend on how "bad" it is. A slight pucker can often be corrected by dampening the backside of the canvas with a water/alum solution. If the canvas is just a bit loose, it can be "keyed up" by tapping in the corner wedges, but this should only be done with the piece in a frame to insure that re-tensioning doesn't knock the chassis out of square. Anything these small adjustments can't correct requires pulling tacks to re-stretch properly.
Re/ RSG as a size for raw canvas, it's bad practise to saturate canvas with a full-strength glue solution, because it embrittles the canvas, and exposed glue on the backside could attract pests (like book-worms and silver fish) and a coat of glue is more hydroscopic than canvas.
The sizing is a barrier coat between the fabric and the priming, and its sole purpose is to prevent canvas woven from natural fibers from absorbing oil and vehicles from the primer and subsequent applications of paint, which will cause it to rot within a relatively short time. Nothing more. For this reason, the proper way to size a raw canvas is first to give it a "drink" of half-strength glue, which will prevent application of the full-strength coat from saturating the fabric. The full-strength glue should be applied when it is cool almost to the point of gelling, so that it will not wet the canvas to the point of saturation, and as it cools on the surface, forming a gel, should be rubbed in with the palm of the hand to even out the coating.
All this takes far more time to explain than to do!