CR,

I think using fiberglass or plastic tub will be a better idea. I've seen and know people does that to their balcony and 3 out of 3 that i know leaks. Waterproofing paint may do the job but also release poisonous chemical to the water, so have to be careful of this issue.If you wanna do a 100% DIY, why not build it with waterproof plywood and line it with fiberglass resin. It is easy to do and 4-5layer of the fibreglass resin will make sure there's no water able to penetrate the wood. Furthermore, plywood is very easy to work with.


Cheers.

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Japanese Ranchu is another ball game