There could easily been old aluminum or copper wiring inside that wall that provided the path of least resistance the lightning was following. Or the water itself might have provided that. Pure water doesn't conduct electricity very well but dirty water filled with conductive minerals is one of the best conductors around. With the added bonus that the water under such a massive jolt heats up and turns into steam.

Water increases in volume by 1,700 times at standard temperature and pressure

So yeah it doesn't take much water turned into steam to fill up a small space like that gap between those walls. Too much and it would look like a bomb went off. Just enough and all it would do is apply pressure to an already weak wall and make it pop outwards just like what we see in these pictures.

The paint probibly sealed it up nice and tight so the pressure had nowhere to go.