Plastering window reveals

Good day

I would like to plaster various window reveals. A picture is attached. The plaster will no longer be visible later (wooden facade). It is mainly about a clean surface for the window.

I would like to mix the plaster myself. A trial with 3 parts limestone cement and 14 parts washed sand gave rather moderate results. Unfortunately, I do not know how much lime is contained.

According to an expert, the original plaster is a cement plaster that still contains lime for better workability. Unfortunately, I have not come across really conclusive advice on what is right in my situation. Lime or hydrated lime? How much cement? How much sand?

Substrate: mostly red brick. Occasionally concrete (old window sills).
Is a bonding agent necessary for the concrete?

Could someone help me?
Many thanks in advance

Michael

I believe the bricklayers here simply used mortar. It probably isn't necessary anymore anyway.

I don't know the exact mixture, but use Kallzementputz. Preferably sack goods, it's not that much. Yes, you can also use mortar for walls and plaster. But you have to have the exact mixture 100% and also do the plastering yourself. I once tried and failed miserably.