They can be - it is just rust, and like the rust on metal we all are familier with, it can vary greatly in depth depending upon how long and and how bad the storage conditions were. I've seen P-Tex oxidation just a couple of months old reach all the way to the core - and I've seen P-Tex oxidation a couple of years old only read a couple of millimeters into the base. Pretty unpredictable. With a bright light, magnifying glass and careful use of a strong sharp probe you can find out how deep it goes before wasting $ on a deep stone-grind. Probe in spots as far from the edge as possible, and ideally at the tip and tail areas which are raised. But of course if the oxidation is only in heavy use areas, you'll have to probe there.
After all these years, I'm still bad - I often don't think about sticking on storage wax until about August . . . and then I just think "oh heck, why bother - we are almost to ski season!"