If you buy prepared canvases for the art supply store and just start painting, you could wind up with an inferior result. Presently, canvases are sprayed with gesso with a high power sprayer, leaving a uniform, but very thin layer of gesso. Gesso in the far past was made of powdered chalk and rabbitskin glue. Today, it is made with more modern formulations including acrylic.
Should you bother applying your own gesso? I think so. The finish of gesso contributes to the life and texture of your painting. I apply up to 5 coats of gesso, using a very fine texture disposable paint roller. The resulting surface provides a “tooth” making it easy for paint to adhere. I have used gesso in a spray can that gives a different tooth, but be prepared. It’s messy, goes everywhere and has to be applied outdoors with your board or canvas inside a box. And you’d better have a really big box. Some of my friends apply gesso with a very wide, very fine brush. The first coat is applied in one direction, lightly sanded and then the next coat is brushed on in the opposite direction and sanded. This is the traditional method and again, very messy. Still, it’s worth the hassle. Some prepared canvases I purchased that had been made in Asia had such an overwhelming smell that they gave me a headache and after a while, had to be left outdoors and were eventually put in the trash.
Can you apply too much gesso? I’ve always been told “no”. But let me share my personal experience. Many coats of gesso can really increase drag on the brush and the paint can grab so readily that you don’t get that lovely fluidity of stroke that attracts so many people to an oil painting. If you are going for a stroke that is more staccato in texture, then lots of gesso may be just the ticket. Paintings that have a lot of geeso priming are easier to transport after painting as the paint sinks in so quickly, it smears less. More layers of oil paint will be necessary, so the finished product look rich and not dry and crumbly.
Experiment! You’ll soon find how much is just enough for you.