Salt 'grinding' play doh

[Netix]

Okay ladies, I have this playdough recipe that calls for salt. You know, the sort that when you bake turns hard and you can paint it. I remember this sort of dough being quite coarse, and I want a finer one.

I read the other day somewhere on the web that you can put caster sugar in the food processor and it becomes fine like white sugar (or the other way around, I never remember which is which). Could I do this with salt to use in this mixture?

Mods, I know this is a bit more of a crafty question, but I thought it relevant to food too. Move if need be.

ETA, actually if someone has a tried and tested baked playdough recipe, that would be fantastic. I need it for playgroup tomorrow, and I want to make hand/foot print plaques for Christmas for the parents to bake once they get home. I'm after a good dough that will hopefully last a long time after its baked! Anyone? Please?