Re: Quick and easy way to prevent oils from bleeding through?
The traditional substance ("size") is rabbit skin glue. You can also use white glue like Elmer's. Art supply stores sell white glue that's supposed to be better Ph balanced, but I have my doubts about that. Make the first coat wet enough to soak the canvas a little. Dry, sand, repeat. And again.
You must seal out the oil completely. If it gets into the fibers, they will rot.
I don't like or trust acrylic "gesso." Real gesso (plaster of Paris) is for stiff supports like panels only, not for canvas or paper.