I thought sandbox sand was not good to use. Also, all the professional places I visited used a pressurized ventilation breathing system, not a mask. That stuff is dangerous and doesn't come out of your lungs.
They used sand on mine and if done properly there is no warping. I seem to remember that the sand won't cut through the undercoating under the car very well. It needed to be rough stripped first. Getting the sand out of the nooks and crannies isn't that bad if the car is on a rotisseri.
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