Well, it seems we all have our methods to get the red phos off the strikers. And that's ok. A few things I'd like to add.
-After the red phos comes off the striker there is another layer of gunk before the paper layers become exposed. Most methods of aggitation (scraping/marbles) will remove that glue layer also which is a bad thing.
Also heavy agitiation like that gets more bits of cardboard in your mix.
-Soaking the strikers in strong Hcl before the red is taken off will always produce those mushy little bits of cardboard. The Hcl removes the cardboard but still doesn't actually take the red off the striker! Furthermore, removing the cardboard while still leaving the red on that little fine strip ensures all glues need to be delt with. Save the Hcl soak until you have a RP powder.
-Moreover, diluted alcohol works best aim for 70% alcohol (which is easy to buy), dilute your alc with H2O. Get a TRANSPARENT container like a plastic pop bottle, add the diluted alcohol and strikers and shake GENTLY, stopping every few seconds and using the transparent container to observe progress. In minute or two the rep phosphorus is all off the strikers and if you shake gently and observe whats happening with a clear bottle, you can stop before the second glue layer get disolved to any signifigant degree.
-Pure water should never be added to red phos it is the major cause "floaties" if you need to add water for any reason dilute it with alcohol.
-Finally, as a test I always let final red phos dry undisturbed, then it should be a loose powder NOT A CHUNK that needs to be broken up.