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I would think that if you want to use sand you need to sift it first to gather only the small grains. That way you can make sure that you have a consistent "medium" similar to salt.

You could also use a hot melt glue pot to store and heat the sand in. Probably less power consumption than a stove, easier on the relationship with your significant other, easier to have in the work area but only practical for small pieces.

I'll definitely have to try the sand as I know that this is what my optometrist uses to shape the branches of my glasses. So I guess it's pretty safe. But I will definitely sift it to have a very fine texture.

As for as water soluble, I don't think it's a problem because if I can't brush it off, it's embedded in the plastic which means it was too hot.

To create the pitted and rusted effect, salt is definitely the way to go though as there you want it real hot so there is a chance that salt will be embedded in the plastic. Easier to dissolve it in water than to pick out grains of salt off the model.