In 2004 we treated these Cortright shingles with the best material available.  See this page for this steps involved.

In 2010, after a harsh winter, we topcoated the green acrylic that was beginning to chalk (as green acrylic does) with an early version of Wearcoat.  Except where an overhanging tree has begun to damage the application, the roof looks as it did five years ago.

Closer look at the shingles