I have an outdoor patio set that I would like to repair. The Parsons chairs are covered in Sunbrella fabric and lined underneath with a waterproof fabric as well. The bottom of the chairs have a material that is dry rotted. It is sponge like and used to be lighter in color. I am unsure of it’s purpose or what it is. Can it just be removed or does it need to be replaced? If it needs replacing can anyone tell me what it may be and how to replace it? Thanks!