ProLong Gold Antifade Mountant hardens overnight at room temperature. For short-term storage (a couple of weeks) you do not need to seal the edges of the coverslip, and the sample should be stable. Beyond that time, though, some dye conjugates will have an off-rate into the medium, so cold storage is recommended. Sealing the edges will prevent long-term discoloration (golden color) from developing around the edges of the coverslip as the anti-oxidants oxidize.
The edges may be sealed with melted paraffin, VALAP (1:1:1 vaseline, lanolin, paraffin) or epoxy glue. Nail polish is not recommended as various components of nail polish may diffuse into the mountant and quench fluorescent dyes.
Find additional tips, troubleshooting help, and resources within our Cell Imaging Support Center.