On Monday I started a painting on canvas I'm about halfway through, then I was reading something on web yesterday and read your supposed to prime canvas with gesso to get a better finish ... I haven't done. Just wondering if they're is anything I can do to rectify this at the end? Can I apply a glaze or something. I'm pleased with how the painting turned out so I don't really want to start over but don't want it to start flaking after while or something, any experts out there to advise me please?
Maybe a bit late now, but - if your canvas was already white when you bought it, it was probably already pre-primed with acrylic gesso. Most canvases bought from art or craft shops are. Sometimes the gesso layer is quite thin so you can add another layer of acrylic gesso (and let it dry!) before you start your painting though it's by no means necessary. You can't really add acrylic gesso once you've started painting as you will then partially erase your image.
The reason for using gesso before painting is to stop the paint being absorbed too deeply into the canvas and to provide a good working surface that the paint will properly adhere to. It will also stiffen the canvas and so support the paint film and so help stop it cracking which it could do over time. We're talking over a period of decades here!
Also, be aware that there are two kinds of gesso - acrylic gesso which is designed for use with acrylic paints; and the oil and chalk based gesso that was traditionally used for oil painting, particularly on wooden boards. Just a bit of extra info if you're wanting to look into this further, don't worry about it too much. If you're painting with acrylic paint, then you need to use an acrylic gesso.