I have been using 100% apple cider vinegar in all the tanks I have been restoring. One tank was probably the worst case you will ever see, and it came out beautiful. My steps are:
1. First shake out all the loose scale and rust and crapola as much as you can.
2. Remove the petcock and try to take off the any filters/pipes that might be sticking up. (So they wont be in the way and get abused when the metal washers are bouncing around inside). Clean petcock well, put some oil on the ORing, and put back on. Be sure to turn to "Off". As an added precaution, I attach a rubber gas line and keep it wrapped up above the tank in case the petcock leaks and you loose all the vinegar.
3. Use water to flush out as much debris as you can (before putting the petcock back on).
4. Add 100% apple cider vinegar to the tank, and fill to almost the top. Best to warm the vinegar on the stove. Then, make a spot which has direct sunlight most of the day, and place tank there on a soft surface so you dont scratch the bottom edges. May need to use some blocks of wood to get tank level. Put top on so vinegar does not evaporate out or bird crap falls in.
5. Let sit in the hot sun so the vinegar stays warm (and the 3% acid works better warm) for 4-5 days (depending on how much surface rust the tank had). For a tank with light rust, try 2 days, and for one that you thought about tossing into the dumpster, try 5 days.
6. Pour half the vinegar out into a 5 gallon plastic bucket (you can save this and reuse in another tank, or for rusty bolts). Add about 50 small washers (about 1/2 inch). Then shake real good for as long as your arms will take it. Try sideways, up/down, etc......shake, rattle, and roll.
7. Dump out the vinegar and rust particles into another 5 gallon bucket. Use a flashlight to look inside and judge if it needs to soak more, or if it is good.
8. If all looks good, continue to add water and shake around with the washers, and then dump out.
9. Then add a big cup of Baking Soda to the tank and fill 1/3 with water, and again shake as much as you can with the washers inside. This will further clean and neutralize the acid.
10. Dump out the water/baking soda/washers into another bucket (not the vinegar buckets), and flush again with water. Keep shaking until all the washers are out. For tanks with the flange that goes straight down a few inches at the opening, those washers can be hard to get out.......i found that a magnetic pick-up tool does a good job at retrieving the washers.
11. Then remove the petcock and let as much water drain out as you can, tilting the tank at different angles to get it all out
12. Then take tank to bench, and use a hair dryer to blow warm air into the top, while water drops are forced out the petcock hole. Be sure you tilt the tank to get every drop of water from the other side of the tank (opposite the petcock). Takes a good 5 minutes to blow dry the tank.
13. Then immediately add a gallon of diesel fuel, or motor oil to the tank (after putting the clean petcock back on). This prevents any flash-rusting until you are ready to use the tank.
14. When you are ready to install the tank and use again, just drain out the oil, and rinse with some gas.
Also, its a good idea to put a coat of wax on the outside of the tank to help protect the paint, in case you spill some vinegar onto the paint. When you do spill some, or if the sun heats up the vinegar causing it to expand and leak out the top, you will want to use soapy water to rinse off.
Vinegar is super cheap, and has worked great on all the tanks I have done.
Use a funnel with a filter screen, and pour the vinegar back into the bottles, and you can use it again on another tank, or for rusty parts. Waste no, Want knot.