Got a couple of new cement ongs. I thought would maybe need to fill first time and drain but old guy reckons just fill, skim any crap off the surface of the water and add a bottle of lao khao haha. Anyone heard of this? The outside of the cement is wet like it has absorbed water. Is this normal?

If you're not going to drink the water then I'd follow the expert advice (Lao Khao kills all known germs).

Any concrete tank will weep when first filled and may continue for a long time, treat it as an advantage as the evaporation will keep the contents cooler

Want to drink the water if possible. We've been buying our drinking water in the 20l bottles and lately it's been tasting like klong water. Peoples obsession here with money will drive them to take any short cuts.

The concrete tank manufacturers don't seem to recommend any washing, but I have no idea what they do when the tank is constructed.

Anyway, I would not drink the contents until the water has been changed a couple of times by usage (to avoid waste).

What's the source of the water?

You should probably be considering some form of filter anyway, make it an RO system for drinking and that will take out most of the nasties that may enter.