Q Cells (glass bubbles) are a fine, light-weight, white powder added to polyester and epoxy resins to make light-weight, sandable, fairing compounds. To make your own fairing compound, mix the glass bubbles in with the resin to a thick gravy consistency. With epoxy systems, resin and hardener must be pre-mixed thoroughly before adding fillers or additives. With polyesters, catalyst (MEKP) should be mixed after adding the fillers or additives to the resin. Remember that the clock starts ticking as soon as you add the catalyst.