Not familiar with anything that would be sprayed on the rotor - I know of tacky spray coating that are sprayed onto backside of pads -

Anyway, at the dealership in the '80's (yep, back then), we had a brake rotor lathe that wasn't a lathe - it had a small abrasive wheel, instead of a single edge cutting tool, that ground each rotor face while the rotor turned in the lathe. This left a non-directional machined face on rotors, instead of a record groove finish, & worked very well to prevent squealing/brake noise.

Of course, back then (& probably still today) MB recommended rotor replacement instead of turning. This just makes sense, as cutdown rotors are less efficient at heat dispersal & easier to warp.

bnc