Plastics oxidize due to ultraviolet rays, oxygen and environmental influences. Even unused skis will succumb to this ageing process. Wax protects the running surface especially during the transportation of the skis or during the summer break.
The season may not be over yet but here are some tips on how to care for your skis after your final ski run. The most important thing is to dry off the ski. Then treating the running surface with Express 2.0 liquid wax would do it good.
Another tip is to take your skis to a specialist store and have them sharpened. Once this has been done, you can treat your skis with hot wax every now and then during the summer months and store them while covered with the wax coating. Then remove the coating and brush off any residues. Repeatedly coating your skis with hot wax makes them fast and is a method favoured by professionals. Use Hot Wax NF or Hot Wax LF red for this.