Even if you had a sealer that would last for 7 years, you would still have to clean the deck after two or three years. Maybe sooner depending on what area of the country.
I think it is not in your best interest to offer such a product. We need to see each customer every year or every other year, to build up business. By the time seven years rolls around, they will forget who you are. My 2 cents..
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Mike Hughes
Everclear
Souderton, PA