ᐅ Is it possible to plaster over expanding foam insulation, and if so, how?

Sorry for the possibly silly question, but is this possible? I often read online that it is, but there are no detailed explanations. For example, can you apply plaster directly onto the expanding foam (roughened surface, not smooth), or is it necessary to first apply a primer or some other bonding agent?

I can only tell you that we applied gypsum filler directly on it, and it was no problem.

That depends a bit on how large the area is where the expanding foam will be plastered. If it is just about filled cracks or similar, it can simply be plastered as is.

The exterior area is the side reveal between the front door and the stair tread...

In terms of size, both the left and right sides are about 3 cm (1 inch) high, 20 cm (8 inches) long, and 1 cm (0.4 inches) deep.

The recess for the stair tread was a bit too large, so I first had to fill it with expanding foam.

I would additionally coat that with a waterproof slurry. Construction foam is not waterproof.

I might consider removing the expanding foam and using acrylic instead. The door frame will have a different expansion behavior compared to the masonry, and it is also subjected to vibrations from the door slamming shut.