If your handwashing you really can't do wrong in my book. I have a black roadster and I run it through the automated car wash until clean. Normally 2 or 3 times- on a somewhat weekly basis.
Though I do not recommend this to any one-my black 05 is still like a mirror when I use the mequires 3 part system previously spoken of.
I hand wax it about every two months.
If nothing else my input may lend some insight as to the amount of durability your finish has. Mercedes grade application and paint were carried over by Diamler on the crossfire project.
My dad still has his first mercedes 71, and there were no fancy cleaning options back then. The paint is still a brilliant silver.
Use a car cover when possible-The sun will do the most damage. U.V. rays overtime attack the molecular composition of anything given a synthetec hue. That's why anything left in the sun fades. Except my girlfriend.
So relax and enjoy my good friend, that's what its all about.