In what order to repaint/fill this stringer

[redhexagonal]

So I have a stringer that was all cracked and had a huge smear of dirty cailk down it. I cleared the caulk off and scraped out the gap, and filled the void with expanding foam so the plasterboard isn’t flexing any more. I was going to easifill the gap and then paint the stringer, but I’m not sure whether to prime/undercoat the top edge first (i scraped it right back because the paint was all loose.)
What order should I prime/paint and fill? Will I be able to get away without caulking or is this doomed to failure? The stringer is cut into the wall at some points so it could be tricky to fit a trim over it. i’m hoping the foam will stabilise it enough so it won’t crack

 

[Mr Chibs]

Just and idea... Perhaps a small section beading may be better bet you can glue or pin it onto the stringer, then just caulk along (if the wall is out) then paint.

 

[redhexagonal]

Just and idea... Perhaps a small section beading may be better bet you can glue or pin it onto the stringer, then just caulk along (if the wall is out) then paint.

The problem is the width of the exposed stringer varies quite a lot, being from 0-15mm cut in to there being a 20mm gap to the wall (i will put some trim over the section with that large gap)