A stamped surface isn't flat. It's full of pattern lines, ridges, and texture, and a sprayer coats that terrain unevenly: thin on the high spots, pooled in the low ones. 

Sprayed coats sit on top of the surface in a fragile film, and they're a big share of the failed reseal jobs we get called to fix.

A brush works the sealer into the texture at an even depth, where it can bond to the concrete.