Seven years ago we used "pretreated" Eastern cedar shingles to cover our new coastal house. The pretreatment was described as dipping that would hasten the aging to the silvery grey found on Nantucket homes. This has not happened. The shingles have aged differently in different areas, primarily turning dark brown where exposed to sun. The company told us to spray on a household bleach solution with 4 to one ratio, water to bleach, leave on for 10 minutes, then rinse off. Doesn't work. It did take off the pretreatment remaining in areas, showing light brown wood, but the tannin in the more exposed wood is still dark. How do we get that silvery grey natural look?