There are several products, the one that coats the tank is called "Creem". It is a 3 step process. Another product is "Rustol" , a one step process.
When you get ready to do it, order a new fuel petcock before you start. Remove the old petcock (it is probably shot). Plug the hole where the petcock was with a wooden plug. Use about a cup of acetone to loosen the old varnished gas that has settled in the bottom of the tank. Careful don't get the acetone on the paint.
Drain the acetone into a container (wear golves and drain it outside or in a well ventilated area). The combination is a nasty smelling stuff. Then use the garden hose with a nozzle to blast out all the loose rust and debris. Flush from top and bottom (through the petcock hole to bubble out the big stuff).
You can also put rocks are nuts and bolts in the tank and shake it to break loose the rust. Count the rocks or bolts to make sure you get them all out.
Now use the Creem or Rustol and follow the mfg directions.
When you are finished put in the new fuel petcock.