I use 303 Aerospace Protectant in a spray bottle on my roof every 6 months. It claims to protect like SPF 40. I buy it from Amazon for around $25 a 32 oz bottle. But that lasts me for a year or more.
Before I apply the protectant, I hand wash the roof with water and a soft cloth to get most of the dirt off. Then, when the roof is dry again, I use the 303 and paper towels to hand apply until I work my way back to the ladder again.
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