I got the tank off my van for the first time. I definitely have to replace
the valve.
I snaded and brushed all the rust off, and it looks awesome. My mechanic
emphatically suggested that I paint it white with rustoleum (brush)
Same for the protectors plate.
But I hate to do it. It looks really cool.

Is there any real bad reason for not leaving it in steel, with several
coats of varnish or other finish?

Steam punk, or good sam club?