Bilt Humber underseal was my choice on the trads. It didn't harden and crack, maintaining a bit of self healing. It washes off with white spirit. Mask off areas to keep clear then spray on.
Autoglym vinyl/rubber cleaner. Renovo do kits for rejuvinating vinyl roofs. The awkward problem is sealing the sewn seams. Beeswax or seam sealer worked. My roof was tight and the thread holes were the weak spots in heavy rain.
