Reinforce outer corner

[Cassandra]

Need advice from someone knowledgeable. I'm going to repair damaged outer corners on a plasterboard wall. I've bought self-adhesive corner guards, but it turns out they can't be used on wallpapered and painted walls. Has anyone tried and knows if it might still work? I plan to fill and paint over it.
Is there a big risk that it will look bad if I just paint over it? I could put wallpaper and then paint to hide the unevenness better.
I've tried searching for previous posts but haven't found what I'm looking for. I really want something self-adhesive, not messing with glue. I gratefully accept tips and advice.

 

[Appendix]

It will look terrible if you just paint.

Do a proper job. Trim away the wallpaper where the trim will sit. Sand down so it becomes even. Attach the trim. Apply at least three layers of filler. Further out from the trim for each layer. Sand Paint.

 

[Appendix]

Expect the trim to "build" 2 mm with the corner. That's how much you need to raise with the help of filler.

Plan to repaint both entire walls where the corners are.

 

[Cassandra]

It will look terrible if you just paint.

Do a proper job. Cut away the wallpaper where the trim will sit. Sand it down so it's even.
attach the trim.
apply at least three layers of putty. further out from the trim for each layer.
sand
paint.

Thanks for the response

 

[Cassandra]

Expect the trim to "add" 2 mm with the corner. That's how much you need to raise with the help of filler.

Expect to repaint both walls where the corners are.

Thanks for the advice, I'll give it a try and see if I'm satisfied.