Hi Filip,

Sorry it took me a while to reply. I hope you found a few topics in Fresco Forum on magnesium.

15% magnesium is fine for ornamental plasters, also marmorino, scagliola, etc.
It is also fine for "fresco undercoats" including arriccio. It will set faster and stronger.

However for intonaco I would not advise to use magnesium lime, because of the fast setting - you will have problems painting detailed giornatas. It will "lock" faster which will cause poor adhesion of pigment as well as while painting pigment (paint) will "slide" as you add layers.

iLia
http://FrescoSchool.com