I have a ~1 sq meter wall section on my patio that somehow had water damage from behind -- the paint on the area bubbled out and was easily removed (almost in one sheet) with a putty knife. This is a condo that I had rented out for 2.5 years, so I'm not sure what the root cause was. The previous 5.5 years I lived there, the original paint job (which I did) of primer + semi-gloss latex had held up just fine. It's a typical rendered red brick wall separating two patios and doesn't get hit by rain and there's no pooling of water at the base that I've ever seen, so the cause is a mystery. I'm willing to just paint again and see if it holds up

I have some "Classic TOA Super Acrylic Alkali Resisting Primer" that I applied after going over the area with a wire brush and wiping with a damp towel to remove the dust. I waited for a couple rain-free days before removing the old top paint, and waited another day (also rain free) after wiping down with the wet rag. The primer is drying as I type this.

Is "less is more" a good motto for primer? I used a small roller and although the entire area is covered with primer, I wonder if it's too thin of a coat? Should I apply a second thin layer of primer, or is it adequate to just make sure all the bare concrete is covered? Or, is two layers of primer actually a bad thing?

Probably not easy to put on a thin coat with a roller, so you should be good. But, if you have plenty of primer, 2 coats can't hurt and a lot of pros will do that anyway.