When you paint a room, you apply a coat of primer. When manicurists paint fingernails, they apply a base coat before adding color. So why wouldn't you do the same with your canvas-even modern canvases have texture that gesso will even out
"Cover rocks with mirrors and they look like melted disco balls." I don't think these are rocks but if you could find a "melted" looking rock you could cover it it pieces of mirror or CD's to create a "Melted Disco Ball".