Neil Careful the ferrocement can become an addiction. I first used it to build a 16,000 gal. fish tank and planters. Then rain troughs and catchment basin tank. Lately some large sinks. Also a waterproof floor for my new
compost bin to prevent seepage to groundwater. In my mind is a relaxing tub for 2. I've found a layer of thinset inside helps to seal more waterproof. Can also be sealed with parrafin or pool paint. Not sure about the colder climates. Mine just went through 2 earthquakes one 7.6 turned tank into a wave pool. So far so good. Best to keep the cement from drying for 30 days. I fill with water after a couple hours here to set up, also wet burlap sacks and plastic to hold the moisture. I use 3 parts good sand to 1 part cement. They say to work it fairly dry, I still have to work with it wet. Best Wishes!!