Sealing the outer grid of a decentralized ventilation system

Hello everyone,

I am currently finishing our decentralized ventilation system.
The outlets to the outside are located in the window recess and will be closed off with stainless steel grilles. Since the plaster cannot be chiseled out completely cleanly so that the grilles fit exactly, I have to make the break-out slightly larger.

My question now:
What is the most reasonable way to seal this?

I was thinking of using white acrylic sealant? Does that work?

Greetings from Lower Bavaria
Joe

Acrylic is not suitable for outdoor use because it quickly starts to mold. I would tape off the grid and carefully plaster it with the top coat. Then paint the entire reveal.