Sure is. By the time the car gets to you, it's spent 1 month+ sitting on a ship with salt air and all that, plus another 1 month+ sitting at docks and holding grounds, usually in industrial areas etc. So you should definitely clay the car before doing a sealant. You'd be surprised how much comes off into the clay bar 
Edit... oops, Frostee got in first. I'd do the same, but swap the dishwashing liquid for APC

Edit... oops, Frostee got in first. I'd do the same, but swap the dishwashing liquid for APC

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