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it is called Yupo https://yupousa.com/what-is-yupo/, about the properties: https://paintingdemos.com/painting-with-watercolors-on-yupo-paper-a-few-techniques/
searching availability here, found similar at Gerstaecker: https://www.gerstaecker.de/LANAVANGUARD-Universal-Malgrund.html
otherwise via Amazon or EBay. If imported from Blick in the US, you pay duty and tax plus shipping that at least triples the price. https://www.dickblick.com/products/yupo-watercolor-paper/

Great information, thanks Otto!

not all of it.... but some. I guess he will be joining us in Steemit soon also, it seems.

Thank you Otto for the research. I did correct the name. I just took the name verbally from Akira but didn´t look at his blog as we are surrounded by his original pieces here. Thanks for that Otto! ;-)