Polyurethanes are moisture sensitive materials.  Polymers that are exposed to moisture may form an in-dissolvable skin on the resin surface.  The skin can be removed and, if no additional particles are noted in the material, a small amount of resin and hardener can be mixed and cast to ensure that the remaining resin system is usable.  However, if the moisture contamination is severe, the polyurethane may become foamy in appearance, indicating that the product can no longer be used to prevent moisture contamination, opened containers of resin and hardener must be resealed carefully.  To begin, clean the lip of the container with a rag or paper towel to remove any material residue.  Next, spray dry nitrogen into the open container.  Finally, replace the lid ensuring a tight seal.