I think most will bead blast them with glass beads. I'm not sure what other media can be used, but I would assume some abrasive powders should also work. Hopefully someone else with some practical experience will chime in soon.
bead blasting for sure. I have a number of plates on hand and could sell them on an exchange basis (I think I even have the clutch plates on hand). You can probably get them blasted locally for cheap too. Looking at $25 exchanged plus shipping ($10).
Make sure that that your steel plates are truly FLAT. Putting two of them together and squeezing the edges or using a piece of glass to chech that the steel has not warped. Otherwise it's new plates.
Lew