Enzie,
If you have oily spots showing on the back of the canvas, the paint is already soaking into the canvas ground, and Michael is right, it will rot. If the Liquin seeped through, so did the oil. In fact, even properly primed cotton canvas can show signs of decomposition quite quickly.
Nothing you apply to the back will adhere to an "oily" ground or help in any manner. The purpose of a glue or acrylic primer is to seal the fibers from the harmful affects of the oil; if the oil is showing through the back, it has penetrated the fibers, and decomposition is just a matter of time.
When I first read your post, it was unclear whether the ground was ever primed. I'm sure you now know you cannot paint in oil on raw paper or cotton or linen; the fibers do not hold up to oil exposure. If you cannot remember the history of the canvas, better safe than sorry; start over.
"Foxing" might also be the culprit; small growths of mold bloom on the surface of improperly stored or primed surfaces.
Check out the thread Chris Saper started on
"Putting failures to work" for great suggestions on what to do with those old canvasses which for whatever reason are no longer viable.
Threaded Mode
