Can I apply microcement on Venetian plaster?

Venetian plaster is problematic mainly if it is polished, waxed, or has poor cohesion. A glossy sealed layer must be removed, roughened, or treated with an adhesion primer. I would not apply microcement blindly over the entire surface. First, verify the cohesion and behaviour of the substrate, as microcement is a thin finishing layer and will not conceal a poor substrate. Use a suitable adhesion primer afterwards.