Base wax provides extra durability for the grip wax.
Swix recommends using base wax no matter which type of grip wax you use. This layer is applied after you have rubbed the ski sole, and ensures that the grip wax endures longer ski trips.
Base wax for hard grip waxes are available both as green and blue hard base wax, which are heated with iron, and in liquid form.
Base wax for soft grip wax (green tube) is available both as regular soft grip wax in a tube and on a spray bottle.