Hi Matita

Some time ago I worked with liquid emulsion (Rollei Black Magic) as well; I tried different procedures on surfaces around 1.5 sqm.
Foam paint rollers, brush paint rollers, sprays, brushes. I achieved the best results using large, very soft brushes. Sprays tended to form bubbles.

I used as well diluted emulsions up to 20% spirit / 80% emulsion, maybe at greater dilutions the bubble-problem can be avoided.

I cannot remember how much liquid I applied exactly, I guess 50-100ml /sqm. Of course it depends on the desity you want achieve. If you apply a very thin layer the picture will be slighlty transparent.