I've been reading things about the coat of plaster "absorbing" the pigments in different ways at different times of the painting process.
My own experience alternates between frustration and satisfactory moments, sometimes my plaster is very "hungry" and absorbs the color rapidly, sometimes everything remains at the surface and my work gets sloppy, I often have to wash it and start again.
I have the feeling that the reel fresco is achieved when the pigment is locked behind the layer of dry lime, when you cannot see traces of brush strokes, when your paint appears to be like a mineral. I can reach that point only if I "squeeze" my painting in the plaster at the end of the work session, I sometimes damage details of my work that way, but the mineral shinny aspect looks much better, and seems to be less fragile to rubbing..
I would like to be able to find a way to apply my pigments on the plaster that would allow me to not squeeze my work in the plaster, to be able to keep the exact state of my art in all its details, to also have sometimes a smoother finish.
How are you supposed to apply those pigments? Dry brush? (cenninni mentions a brush squeezed between the fingers to get rid of the excedents of water) Are you supposed to allow a big amount of time in between 2 layers of pigments? When the fresco is finished, is it ok to see pigments at the surface of the plaster or does it have to look like(pigments+plaster)it is the same element?
I went to see the frescoes of jean charlot at the university of hawaii, the sand and the pigments are incredibly compacted in one single element, no brush strokes, no "materiality" of the pigments, if you look very close you only see teinted grains of sand.
I guess in that case carbonatation really protects the pigments, would it be the same if the colors would appear more at the surface as independant from plaster? Would they be locked by carbonatation anyways?
Ilia I thanks you once again for your your amazingly precise and helpfull answers. I found some very good and very cheap tuff here, it is already mixed with the lime and ready to go.
Mathieu.

The line from Cenninni is a very important and often overlooked advise. I would say that it should be treated as a rule, not just a suggestion. Especially at the beginning of the day and also closer to the end.

What happens is that the plaster should "suck" the paint of the brush and if brush is too wet the paint would just sit on the suface. You can use a little rug to swipe the brush of the excess of the paint everytime you load the brush, but I found that this way you can not control how much paint is sill left on the brush.

It takes some skill to polish the intonaco before painting, my plasterer spends about 20+ minutes doing just that, however overtroweling is bad, so go figure...

So when the plaster is done right and your brush has just right amount of paint(not too wet and not too dry)
You will get that polished look you want. Also you can buff your work as you go along with dry soft brush< I use guilding brushes for that.

I also add lime (bianco di san giovanni) to all of my colors - this way the pigment would be "inside" the plaster. If you want to have paint without any white, than you need to use a "milk of lime" instead of water for your mixes, I believe thats the name.

Get some(50/50) putty stirr it with water, let it sit for a while. Lime will settle on the bottom and the clear liquid is want you whant. (sit for a while I mean a few month being stirred ocasionally)