I do about 4 coats of gesso and sand each coat (even if your canvas is primed). I use thick professional gesso and put it on with a palette knife. Letting it dry about 3 hours between coats, that will take the better part of a day. I do glazing as needed if I need a color affect. Your painting needs to dry for a year before varnishing. I don't know what a fixative can would be used for with paint. That would be used for pastels.
Bear in mind that if you are just learning, you would gesso and paint. More often than not, you will sand down existing paintings and paint new ones on the same canvas because beginning paintings are not so good. Things like glazing and varnishing will come much later.
