I tried the rust kit from JC Whitney but I never bothered with the poly coating. I mixed up the acid with water- use muriatic acid from harware- masonry section- cleaning bricks. It eats the rust clean and leaves a shiny metal surface behind. My tank sat 19 years unused and rusted. The only thing is that once the rust is off, the metal is unprotected. Fill it with gas right away when dry, after you rinsed all of the acid out with water. Hot sun helps. About a day to eat the rust. Have the tank and pretcock off the bike. You'll have to make up your own plug for the petcock hole. I used some old innertube cut like a gasket and the filter off the petcock and screwed it back on.  
I tried the gravel bit and gave up after two minutes. Besides that fine sediment can do a job in you motor. Pennies are big for the corners.

If the rust goes all of the way through you'll have seal your tank with a polymer to stop the leaks.  I think your could also use battery acid- -sulfuric acid. I've used that to strip steel of rust when I am forging and want to varnish it.

Good luck

TC

just for the sake of throwing it out there, the works is muratic acid.

Ive found BB's or ball bearings and old gas to work best to knock out the rust but make sure to recoat it so wont rust again available at local parts store

Best method I found on the internet was to fill the tank with something that will break the rust apart (sand,rocks,bb's,nuts and bolts), then cover both fill and petcock hole with lots of tape so nothing will come out.  Then wrap the heck out of the tank in thick blankets and such and tape them so they wont come undone (just acts as a cushion around the tank).  Then put it in the dryer on the setting with no heat for an hour or so and let  your homemade sandblaster go to work!--just dont let the wife catch you  Then pull it out, unwrap the blankets and open up the fill hole and dump out the media you filled the tank with.  Out will come all the rust and the inside of the tank will be left clean metal.

I used this method and then I used Bill Hirsch's products for cleaning and coating the inside of rusty tanks and mine to this day (over a year later) looks brand new and works like a charm!!  

P.S.  The stock petcock will probably be useless since the screen will be caked and covered in rust particles.  If you are doing this on a Savage I would recommend converting the petcock to the Yamaha Raptor petcock and never look back.  

HTH

-Matt