Verne,
I'd be afraid to sand the cardboard. I think you'd end up with a suede finish. These panels are pressed paper and their smooth finish comes from the rollers they are fed through. I seriously doubt if you could ever reproduce the smooth finish again without soaking the surface and running it through a pressurized roller system again. (But I've been wrong before.
)
Has anyone tried painting them with different stages of gloss? Mine appears to be somewhere between a satin and semi-gloss finish.