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Bleech white or other tire cleaner and a brush will get it looking better. For dressings, you have to think ahead a little, most tire/vinyl dressings leave the surface slick an oily. I had better luck using instant detailer on the horizontal surfaces and then tire spray on the sides.
Clean it with automotive soap and water. Use a stiff brush and give it a scrub. Let it dry. Then, apply Just Once (justonce.com) or 303 Aerospace. Buy the large bottle, the liner will soak it up with the first application. Apply about 3 times a year.
For a permanent solution, get LINE-X ReNew. The bedliner will get a new texture and then LINE-X Xtra. Xtra prevents fading and you will never have to do anything to the liner but wash it.
BTW, LINE-X ReNew can be used on ANY brand of spray-on bedliner.