I don’t think it needs any kind of sealant. There is at least on brand of “stain” that you can paint them with, Behr. I don’t trust it to stick and think it would look worse if if came up in spots.There probably is something transparent that would give it a fresher look but you don’t need it.

BTW, when I want to clean the fake deck I connect a hose to the faucet on the bottom of the water heater to get really hot water. It is good to remove sediments from the heater too and can make it last longer.