Exhuast Valves: Rough White Powdery Deposits

2009 Ford F-150 XLT
4x4
Modular V-8 / 5.4L (330 Cu In) 3v
165K Miles


So, have the cylinder heads removed for camshaft bearing related issues. As I was cleaning the cylinder heads I inspected each of the valves.

Specifically, the exhuast vales have this rough white coating on them that seems to be baked onto bottom surface. As I was cleaning them I noticed it wasn't coming off no matter what I tried, (Chemicals, Wire brush, Brillow pad). Stuff is hard as a rock. The backs of the valves seemed slightly pitted but relatively ok. The stems themselves are not scratched or rusted.

The truck didn't seem to have valve related issues before the engine teardown.

Question's:
1.) What's the white coating of the exhust valves?
2.) Will this coating create hot spots if reused?
2.) Would it be advisable to replace them?

Thanks for your time!