Rust is not only a conductor for electricity but also a harsh abrasive when it contacts moving parts. It is important to clean the rust from the tank to prevent sdamage to your fuel system and your engine.
Muratic acid is a very powerful acid that you do not want on your skin. Also be very careful of the fumes that come from the acid etching the metal. You do not want to breathe these fumes! Being that it is an acid, you should use a base such as baking soda(mixed with water) to nuetralize the acid after cleaning. Then rinse with water, and last with isoprpyl alchol to remove any moisture.
To do it right you can then seal the tank with POR-15 or another gas tank sealer. POR-15 is about $30 dollars for the kit but it is the best! One kit will do about 20 tanks. The only downfall is that it is a two part epoxy sealer meaning that it is has to be used all at one time or it goes bad. So get together with your buddies and do a bunch at one time to save some money.