They're using Rhino Pipeliner 5000, which freezes as sprayed-on curds almost exactly as it hits the surface. Won't flow out, flatten, and self-level like standard epoxy or paint would. Under standing water, the orange peel texture is a perfect scaffolding for algae, bacteria, and general bio scum This shoot shrinks 1-3% when curing, which in a pipe is exactly what you want. On a big slab, uh, no it will pull towards the center and delaminate from the substrate at the edges.