I saw one a couple of days ago. A private deck on a dune, with a funnel & garden hose running to a PVC pipe draining on to the backside of the dune. I never would have looked for it except I could smell the piss.
I would suggest pulling off what you can from the wall & sand down any major flaws. Find a paint store that carries Zinsser Gardz. Put a coat on the whole wall. It should **** into the porous area. Put another coat or 2 on until the finish is shiny when dry. Once you have that shiny finish you...