Sealing the back of canvases

This practice is almost uniformly condemned today. It is less of an issue of the fibers needing to breath. The more important reasons are twofold. The application of materials to the reverse of the canvas can create brittleness in the case of Japan size. This may also rot the canvas overtime.
Additionally, whenever you adhere something to a portion, as opposed to the complete, verso of the canvas you will eventually see that impression of that material telegraph through to the surface resulting in a topographical bulge in the shape of the applique that can be seen on the front of the painting. You have probably seen patch shaped bulges visible on the surface of poorly restored paintings. I have also seen this happen with gallery labels that were stuck on the backs of exhibited paintings. This is all just the result of the inevitable natural alignment of physical forces that occur when a smaller objects is attached to a larger and flexible object. The same does not happen with lined paintings but that has its own issues.
The tinfoil idea would not be terrible as long as no adhesive was used. Today, we generally just attach a rigid, vented backing board (eg Fomecore or acid free blueboard) to the reverse of the stretcher. This does almost everything positive that the earlier procedures did without any of the negatives. BTW the small vents left in the backingboard prevent the creation of a micro-environment which may occur in very humid environments.

​If you are interested in learning more about how to prepare your own backing board, instructions can be found in our Resources section in the “Storage, Exhibition, and Handling Tips” document.