In the past, I have used mountants such as DIATEX (which is a synthetic mountant of 40% acrylic resin solids, and 60% toluene), or Canada Balsam in xylene for general purpose mountants. If a tight seal of resin forms around the coverslip, the prep is usually good for some time and may last indefinitely. However, the center of the prep still contains resin mixed with solvent that has not had a chance to evaporate, If the solvent continues to evaporate slowly from the prep, large air bubbles will replace the solvent and ruin the prep.