Muriatic acid? Its good for removing oxidation from aluminum but I would use something else to remove rust from a moped tank.
- vinegar (safest one but also least effective and will rust again quickly)
- citric acid (the tank will rust almost immediately again when your done)
- phosphoric acid (leaves a phosphatic surface which will resist rusting)
- some commercial stuff (Evaporust etc. - take your pick)
I use Phosphoric acid to remove rust and IMO it works nicely. Nice thing about it is that it does indeed leave a phosphate coating on the tank sufrace so no problem if you leave it empty for while working on other details on your moped.
Only few days ago I treated my gas tank (petcock in place) with it and no problems what so ever. The 35% phosphoric acid solution didn't damage the petcock or even the paintwork.
Rubber gloves and safety glasses are a must though. Did get a very small splatter over my shirt and its now riddled with tiny holes ;-) (edited)