Not sure what you mean here.
Is waterproof concrete so much more expensive?
Are there regulations that somehow prohibits this? On what grounds?
You mean convention, as in, it's not been widely done…

I'm trying to understand how to build a concrete bathroom.
I've searched all over the net, but can't find anyone describing the process.
I want the whole bathroom in white…

I don't like panels, either;)
I want something like the inside of my bathtub on the walls, like paint.
Can anyone recommend something to paint the walls with something that is…