Thanks for your replies. I searched again for some more information about the subject, and I found some very useful thoughts from some people:
1. You should never apply gesso to the back of the canvas, it needs to breathe.
2. What you really should have done prior to painting was apply more gesso to the front of the canvas.
User: Ron Francis
1. Certainly old paintings that have been sealed on the back have not faired well, but these were not sealed with acrylic gesso rather than sized with hide glue and then painted with oil.
Other users say that the important thing is to prepare the front of the stretched canvas really well.
User: Marcus Lim
1. When we decide to use canvas for oil painting, the most important area to prepare is the painting surface. We all know its importance with protecting the fabric from the corrosive effects of the oil medium. So, preparation with rabbit skin glue or any modern primer is priority. This will do away the need with priming the back side of the canvas, to serve the purpose.
User: Beth Goldowitz
1. If the canvas has been properly primed, either with rabbit skin glue (old school) or acrylic gesso (modern). then that’s all you need do to protect it. Paintings prepared this way last for hundreds of years.
User Kathleen Grace
1. The back of the canvas needs to remain as is - do not prime, do not seal. The painting breathes, and if you cut off the air supply you may end up growing mold between the layers, as well as creating competing sides which will expand and contract at different rates.
User Marc Clamage
1. If the canvas has been prepared correctly, it is not necessary protect the back of the canvas in any way. If I may, the only serious threat to the back of the canvas is the front of the canvas. If the canvas has not been suitably sized and primed, the oil will leak through and rot the canvas. In that case, taking steps to preserve the back of the canvas are not going to do you much good.
So, it seems that I should not do anything to the back of the canvas, and I’ll do so accordingly.
So far, everyone considers leaving the back of the canvas as it is the best thing to do. Thanks to all of you. I think my doubt is resolved. Thanks again.