Now a 60x60cm prototype:
The color is not solid enough. I've not used enough white pumice and the canvas is not well covered, causing it to soak the paint
Lessons learned => use enough pumice to have a solid layer (about 300ml per 60x60cm square).
I've not used a full waved pattern because that was somehow ugly
The mat is very flexible, the pumice does not get off, and while not soft, the surface is not coarse.
Applying the same method with a more solid pumice layer on a 180x120cm canvas and sealing the paint with PVA should made a fine mat