I’m just in the process of painting my 400 tank but still unsure what to do where the petcock (fuel tap) fits/seals to the tank.

If I paint the whole area then the oval seal ring from the petcock is sealing onto paint and lacquer, not ideal IMO.
If I leave bare metal it has a chance of rusting.

What have you guys done and any problems with the way you addressed this area?

Slap a piece of painters tape over the opening, sit petcock on there and trace in pencil. Then use razor to cut out just inside the perimeter of the oval, just a few mm. That way petcock is sealing to bare metal, wouldn’t worry about rust as much as a leaking petcock ruining a paint job!

Replicate how the factory did it, originally that area is painted.

The flatter the area the better. Lower could leak. Higher might be worse? Paint over it. Then' if you are still concerned' - mask the outside perimeter and lightly sand to rough the surface. O rings work as intended. 

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I decided to go middle of the road.......
Etch primed the whole tank then masked off the petcock for primer and color 

I have over 100 hours in this gas tank.  The petcock rubber gasket lives on top of probably twelve layers of primer, bondo, two part enamel and carnauba wax.  Nary a drop of the dead dinosaurs have escaped the seal.  The factory did the same. 

BTW the oil tank is NOS and the banjo bolt and seals are on the original paint from Japan.  They dripped until I added just a schmear of silicone RTV as a last resort.  No drips now.

curious what is that rear fender, i like it.

It is an aftermarket plastic fender stitched into a fitted grey plastic fender. Forty years ago. Picture attached.