There's a 24x30 inch stretched canvas in my studio that could probably serve as a boat anchor. It has been gessoed-over too many times to count. It also sports a thick layer of texture created with a wall compound called Flex-All. This is my Experimentation Canvas and it has provided a working surface for many, many experiments in its lifetime. Which is to say that if it's ever hung on a wall it will constitute a safety hazard. That puppy is H-E-A-V-Y.
It didn't start out to be an experimentation canvas, of course. It started out to be a nice landscape or something (I no longer remember what) that I decided I didn't like. So I gessoed it over. And over. And over. By now I've lost count of how many paintings and half-paintings lie hidden under its many layers of gesso. Some turned out ok, some were mediocre and some were truly awful. All of them were the subjects of experimental techniques: dripped paint, weird glazes, scratchings and scrapings, unconventional substances and who-knows-what-else.
Somehow I've always found it easier to conduct experiments on this one (ridiculously abused) canvas. It's my dedicated sacrificial lamb. I never worry about creating masterpieces on it. I worry more about it falling and fracturing my foot. What I appreciate about it is the total freedom it gives me to try any technique I feel like trying with no concerns whatsoever. No hesitations about trashing an expensive new canvas or blowing through costly sheets of watercolor paper. Nope, I just get out the gesso, apply a coat or two over the previous experiment and voila! I have a fresh, clean surface on which to play.
Gesso is amazing. Slather it on a failed painting and it does away with your sins (an unfortunate acetone catastrophe comes to mind). It's absolution in a jar, capable of returning things to a state of purity. I love the stuff.
Nowadays I use Utrecht's gesso, which is very opaque and thick but can be thinned with a bit of water. Cheap Joe's is another brand I like very much. Both are excellent products and are good values for the money, but even a thin, runny gesso can provide you with the freedom to try new techniques. Just pull out a not-yet-varnished canvas you don't care for, hit it with some gesso -- any gesso -- and you're good to go! Have fun!