"Another obvious visual external difference with the HAF 52+ is the special metallic paintwork called Haveglass 2 which absorbs/scatters some radiation and helps to reduce the aircrafts radar signature. This paint has the texture of fine-grain sandpaper and is very difficult to clean."
From article in Aviation News Magazine May 2009 issue, you can read it at
http://www.aviation-news.co.uk/viper.html
Ab Origine Flexibilis