You can use the gravel orv the nuts and bolts. I have done both successfully in the last couple of years. Ive never tried sand. You have to make sure you wash it out very good. then blow it out good with air . If you have enough air pressure it will knock any scale loose as well as drying it out good. You can use something like ospho . it will eat into the rust and seal it . You always take the chance that you will clean a hole right in the tank but if thats the case you needed a new tank anyway.This week I used something called Right STuff De-Ruster metal conditioner. I am doing a Frame off on my 64. It seemed to work very well and its cheap. I bought it from a local tractor supply house for $4.99 a qt. Goog Luck.