Public service announcements. Please don’t use these.

Besides uncontrolled abrasive dust, the biggest issue is that flat-rate puts guys in a hurry, many don't care. They figure the same as the places they work, kick out the work, if You produce 2× & only 5-10% comes back it's a win.
As far as surface destruction goes, too many times what appears to be gasket/residue on the surface is really staining or contaminated pitting/recession in the surface. They will tip a fresh cookie up on end and destroy that part of the surface trying to "get it clean", careless & stooopid, but all too common.