An 18 elastic is a strong grade of elastic used to hook and land carp. These are poles for fishing lads, not window cleaning, and they will not last very long. All the joining and breaking down of sections will see the joints chip, especially if you're trying to work fast. I would get yourselves down to your local fishing tackle shop and ask for a full set of joint aligners. These are domed shape, plastic attachments that stick in the ends of each section and stop the joints hitting each other and cracking.
But I would definately say that the life span of your new, on the cheap pole will be a short one

Take the plunge and buy a purpose made pole. fit for the job. I paid £62 for an 18ft pole and £212 for a 42ft version, from Varitech in Plymouth
I never use my WFP to catch fish or my rather expensive £1000+ fishing pole to clean windows. It works better for me that way
