A simple shake of the tank will tell you if you have all of the gravel out. Commercial gas tank cleaning/coating kits use phosphoric acid (the same stuff that's in soda pop) to clean the rust & varnish. I used muratic acid because it's cheaper and easier to get. Be aware that Muratic acid is bad stuff and it Will burn you and eat your clothes or anything else it touches so be careful if you use it. About 5 minutes of sloshing the acid around in the tank is enough. I then poured a couple gallons of strong soda water in to kill the acid, rinsed the tank good, let it dry overnight with a fan blowing in the filler neck and coated it with Redkote. Personal experience has shown me that those who have had problems with quality tank coatings haven't followed the mfgrs prep instructions.

Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.