Gas tanks R & R is fairly simple.   If it leaks or rusts, get rid of it and replace it.   Yes you can paint them, but the problem is the inside.   Gas tanks are coated with Zinc.  It is galvanized.  This is for galvanic reaction to prevent rusting.  There are long explanations for this, but the short explanation is the zinc is sacrificed or wears away preventing the metal on the tank from rusting.  Once the zinc is gone, you need to re-dip the tank or get a new one.  Rust happens fairly quickly after the zinc is gone.

Here is an example of my filler tube.  Similar to the tank they are usually galvanized.   This one is 40+ years old.   Looks fairly good on the outside, even better if I cleaned it up.  Most all the galvanized coating is gone.


The problem is not the outside, it is what is happening on the inside. 


I don't want all this rust sediment going into the tank.  Two choices.  Clean up this one and get it re-galvanized or buy a new one.   In this case, the new one is cheaper, much cheaper so on with the new part.

Those new replacement fillers, they come close, but will need some work to align and get the cap sitting right.   In my case the overall bend was off by about 3 degrees.  Good thing the new hose to connect it to the gas tank was longer than I needed.  I needed the extra length to get the adjustment right.