Let's assume (yeah, I know what that means!) that we're discussing stovebolt Chevy rocker arms. They're either cast or forged, not sure which. The only way to "harden" the surface of most castings is with a process known as "case hardening". Us gunsmiths and most blacksmiths have been doing that for a few centuries, long before automobiles came along. That rainbow color pattern seen on some antique firearm receiver dastings is a result of a variation of that process called "color case hardening" It's not much harder than the underlying metal, and it's microscopically thin. A stamped steel alloy rocker like a small block Chevy V8 uses, and a lot of others can be hardened to have a much tougher surface than a casting or a forging. I would never, ever try to resurface a ball-pivot SBC rocker- - - - -once the surface hardness goes away, it would wear very quickly. I believe the "do not resurface" rockers the GMC service manual is talking about are also the stamped steel type. Cast/forged rockers are much more forgiving when it comes to resurfacing than the stamped ones.
Once we get away from the original geometry of the factory valve system with radical valve lobe lift, different rocker arm ratios, and other non-OEM bits and pieces, things really get confusing. I've spent lots of time measuring and tinkering with things like pushrod length, rocker arm angles, rocker tip sweep across the top of the valve stem, lash caps, etc. Those engines usually lasted less than a thousand miles before they either wore out, blew up, or got rebuilt!
Jerry
