Over 10 years ago we had a concrete fountain in the front yard.  We
used pool paint that we bought at a pool supply company.  It used a
rubber base that used a chlorinated solvent (Tri-chlor as I recall).
Nasty solvent, but a great paint for continuous contact with water.  (If
you use this in a closed space, be sure to provide lots of ventilation =96=

the solvent is not good on the heart, lungs, and brain.)  It adheres to
the concrete great, and is not bothered about being below water
permanently  (It lasted over 10 or 15 years in direct sunlight before we
re-painted, so in a shielded area, I would expect longer than that.  And
even then it seemed to be watertight, it just looked a bit chalky on the
surface.)  Such paints are probably still available for pool use, so I
would try those.
    The other solution is to do what is done for spas and tile shower
stalls, and that is to hot mop asphalt on the floor and walls.  It=92s
thick, waterproof, and basically remains a bit flexible for years =96
i.e., it stays tacky for a long time which is an advantage in not
cracking and leaking, but a disadvantage if you have to walk on it.
You=92d have to lay down walkways of wood or tile after sealing the
cistern (or keep a pair of boots near the entrance just for work in the
cistern.)  Just a couple of thoughts.
Charles R. Patton



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