Hi Clare et al

Coval does not need to be reapplied after graffiti removed. Geeky science stuff, it's not an adhesive, it actually bonds molecularly to the substrate - not the paint. I don't know about breathability issues, but they make a version specifically for concrete, so I imagine that this was taken into consideration.

Here's more from the website: http://covalmolecular.com/the-advantages/

The challenge that I'm running into with it is in the event you want to repaint. I have having to get the wall bead blasted to "break" the surface of the coating. Can also be done with a sander/grinder apparently. 

The problem I've had with my limited experience on sacrificial coatings is that they attract dust & dirt. Better for vertical surfaces, but not immune. Conservators like them, understandably, because they don't adhere to the paint and are easily removed, taking graffiti with them. 

As has been stated previously - no easy answers, and no single answer