Do the retaining walls have soil/rainwater behind them? Chicago
experience is that you want/need a breathable primer in those
cases. i.e. gesso breathes much better than polys or epoxies.
There are other techniques (developed over years of conscious
attention) that have been very effective in reducing/eliminating
graf on murals (at least in Chicago). Many of our murals don't
even have coatings on top of the color - the fewer the layers of
paint applied, the better the wall will breathe.
Jon Pounds - HWWODOCPAG
He Who Was Once Director Of Chicago Public Art Group