Lye is sodium hydroxide, NaOH. With both reactions, you'd get salt in solution; my only thought as to why you wouldn't want to go that route is that salt in solution is in equilibrium with salt out of slution, and enough salt may remain on the concrete (especially since it's been pitted) to interfere with the remainder of the painting process. I'm good on the chem, but I'm talking out of my ass on the painting.