You don't have to drain the tank. It wobbles with the sloshing gas once it's off- no big deal.
Petcock off will stop the fuel flow.
The gas line goes on a "barbed" fitting on the petcock. Sometimes there is a small clamp- self explanatory really. Once the clamp is off, you may have to gently rotate the hose before you slide it off- again no big deal.
You mention sandblasting, while it might mean actually sand, I'd be careful. Glass bead blast or the new "soda blast" or walnut shells etc. will be less traumatic to the thin steel of the tank. I'd also give it a real good interior clean when your done- 1/4 full of kerosene and a good shaking would probably do it.