I learnt this the hard way, even cleaning with a soft microfibre cloth scrathes them, only way to clean is with Id huess a powerfull airline if it is dust, or if grime, then use some water/soap and swirl then airline to blow dry so it dose not leave steaks then let dry DO NOT TOUCH A REFLECTOR!!! EVER!
My D40a has this problem but works fine, wish I could get a spare reflector, I have no idea how people who mod lights manage to not damage reflectors and keep it all clean insde, guess it is practice!
You can use your tong, that wont scrach it but er yhea bit gross (Vihn gave me this tip)
There just so fragile, I guess that is one reason optics are so good!