When do I glue plaster decor on the wall?

[SwiftD]

I have cast my own plaster "things" that I will put on the walls, so they become part of the wall, i.e., like decorative moldings but as figures. They are about 12cm in diameter. They will be painted the same color as the wall.

I have read that the best way to attach them is with PL200 or something similar.

Now the question is whether I should glue them to:
The hydraulic lime plaster?
The hydraulic lime plaster that I first prime?
The hydraulic lime plaster that I first prime and then finish with spackle?
The hydraulic lime plaster that I first prime, then finish with spackle, and prime again?
The first layer of wall paint?
When the wall is painted and finished?

 

[JOW]

I would probably say sometime before the first layer of wall paint.

I don't think it matters much as long as the glue sticks.
The priming is meant to bind dust and particles, so after that should work well.
I can add that I've never glued drywall items to a wall so... take it as you will, I'm just reasoning a bit about how I would have thought.