Murphy's Oil Soap is an old favorite. Most glass shops use silicone spray. A couple tips/hints. Be sure the edge of the glass going into the rubber is rounded slightly. You can hand sand with a piece of coarse (80 Grit) paper. This helps the rubber move when you rope/ hook in the gasket. A cotter pin puller (aka radiator hose tool) works to get the stubborn areas. When you"slap" use a cupped palm and steady the glass with light pressure from the other hand. John