I use a 5 gallon bucket, put in only about 1/2 to 1 gallon of blasting grit at a time and add WARM water. Stir it around real good, pour out the oily water, repeat with more warm water about 5-6 times and that has worked well for me. The side of the bucket and my arm were covered in oil (came off easy later with some Dawn dishwashing detergent), but the grit itself was now clean. When I filled up the tank, there was still a very slight sheen on the surface, but a paper towel slurped it right up and I have never seen anything more after that.
I do about 10 lbs at a time in a 5 gallon bucket. Fill with water and stir the crap out of it, pour the water out and repeat 5-6 times. Whatever slick is still there on the last rinse doesnt really transfer to the tank, at least on the four tanks Ive done with it. Only problem I ever had was one time I didnt rinse enough, there was a coating of brown dust that would settle over everything and it was impossible to remove by vacuuming (too heavy)