got a new, overdue, tank for the duster.what is correct paint?flat black?

I clear coat mine, most straps are silver too.

No coating of any kind is needed.

Tanks were hot galvanized like the ones Spectra carries, not painted like the ones that Vans carries.

Yes, hot dipped, but not actually galvanized, which means hot dipped in zinc. Tanks were made from terne sheet which is steel sheet hot dipped in lead/tin. Lead/tin is the same as solder.

"This specification covers sheet steel in coils and cut lengths coated with lead-tin alloy (terne metal, see 3.2.3) by the hot-dip process. This material is commonly known as terne and is used where ease of solderability and a degree of corrosion resistance are desirable. It is especially suitable where resistance to gasoline is required..." From http://www.astm.org/Standards/A308.htm

Tank halves were heated at the flanges to make a soldered joint, same as is now done with the Spectra tanks. Since solder does not rust, there was no need for any coating by the factory. Old tanks eventually rust when long exposure to condensed moisture compromises the solder and exposes the steel underneath.

Tanks left the factory unpainted, but usually with some undercoating overspray even though only the wheel wells were undercoated and the whole bottom was not.

In time, lead/tin or solder will darken from exposure to the atmosphere, so some kind of coating like clear spray or RPM may help a restorer keep the original, shiny look. But there might be some adhesion issues with clear spray on solder.

I coated mine with ECS's RPM, invisible once applied and protects against corrosion.

Well then this might not surprise you. After a 10 year search I finally found an NOS one, 1 day to the hour after I installed and undercoated the (only available AND also INCORRECT reproduction ) in the pic

*I may or may not install it now, as long as I have it safely stashed it's a no big deal down the road swap out.