I recently learned an expensive lesson: don't buy student-grade stretched canvas unless you plan to use it only for practice. Last month I had decided that it was time to tackle a large graded wash in oil. I chose a shot I had taken from our canoe of a perfectly calm surface looking up river. After eight hours of painting interspersed with drying times, I was satisfied with the wash and proceeded to finish the painting by completing the details in the upper half. As usual, I had to flip it upside down to accommodate the limited space in my tiny studio. Canvases are translucent if you hold them to the light and I swallowed hard when I saw several wormy clusters under the painting.
The reason that oil paintings must have an under-layer of gesso is that oil will eventually rot canvas. When I looked at the back of my canvas, I could see not only that the gesso on the front was uneven but also that oil had already started to bleed through. The bell tolls for thee, "Further Up River." All that remains is palliative care at home.