I saw and article on ARC (2nd of june) on sanding paper.
It seams that there are two different systems of defining the grit of sanding paper!
It now finally makes sense to me that some American modellers use 400 grit! This is compareble with what is called 600 grit in Europe.
all in all an very important fact, check the article at:
http://www.aircraftresourcecenter.com/