I got stuck fixing a friends 84 jetta.
Had no power.  Pulled the injectors(CIS), did a spray pattern check.  Two
spraying off at weird angles, the other two nothing.  Pulled the fuel sender
to check tank condition.  Rusty, nasty scale.  Completely blocked strainer
on pickup.  Thats why the fuel pump was so loud...

Anyway, I called some radiator shops to inquire about acid washing the tank.
Prices were 50-75 bucks with absolutely no guarantee.  They really don't
want to touch the things.

I went to Big Lots, a local closeout store.  They carry a toilet cleaning
product called "The Works".  Basically a mix of sulfuric acid and other
chemicals.  69 cents a bottle.  Drained the tank and poured two bottles down
the sender hole and swished around, being very careful not to get any of the
stuff on me.  Had the garden hose handy just in case.

This stuff did a fantastic job eating and dislodging the rust in the tank.
Looked brand new after about five or ten minutes.

Then filled with water and rinsed out.  Used a shop vac down the sender hole
to completely dry it.  Worked great.
The Merkur tank is next...

I'm sure this would work just as well on vanagon tanks.
If you burn, bleach or blow yourself up, don't blame me.

Happy DIYing.

FWIW, Pat.