Make sure you are purchasing plain epsom salt and not the lovely smelling stuff(the scented ones for baths as that may contribute to residue).
I mist with epsom salt all the time and have never had residue on my plants, I have a simple misting spray bottle that I bought at walmart for $2.77, 16 oz, i fill it half with cooled boiled water from my kettle, the other have with water from my fish aquarium, then add two very accurate pinches of epsom salt, sometimes 3 pinches if i didn't think my first 2 were accurate enough. I stir this around until the crystals are completely dissolved.
I mist the entire plant and the top of the soil. Works well for me.