Not all manufactures use the same fabric, or treatment. It depends on how it was treated originally. According to some experts, they are either silicone or Polyurethane treated. It might be best to contact the manufacture. That said, REI recommends Nikwax Tent or Gear SolarProof. To Quote an REI expert:
"Nikwax Tent & Gear SolarProof® is a safe product to use on your outdoor equipment. It is water-based, non-flammable, contains no volatile organic compounds (VOCs) and is fluorocarbon free. Nikwax Tent & Gear SolarProof® provides Durable Water Repellency (DWR) and ultraviolet (UV) protection."