We likely agree that not all of our paintings turn out as planned.  And some artwork just doesn’t seem to sell; perhaps it lacks that emotional connection needed by the viewer.  If the canvas is of good quality, it makes sense to reuse it.  The first step is to lightly sand the surface with a fine grain sandpaper taking out oil ridges where paint is thick and leaving a fairly smooth surface.  Dust with a micro cloth.  It you are comfortable with the results, apply two coats of oil gesso.  Allow each application to dry and then lightly sand the surface before adding the next application.