I too have had mixed results with 845 regarding durability
Note that I *always* find two/more layers to work a lot better than just one.
On the A8's rear bumper cover the 845 didn't last long at all.
On the front plastic (or is it fiberglass?) grill surround of the Blazer the 845 lasted for months on end
No, it wasn't a matter of what it was applied over or the application conditions or the shampoo, or anything else I can think of
Just seems to work better on some applications than on others.