One more question. If I added fresh tufa over the whole planter, will that save it from a complete complete waste?

Serene

Serene, all hope isn't lost--Mist it with water and paint with a thinish layer of slurry (paste made with portland and water only). Cover the crack (and other points you think might be weak) with that mesh drywall tape and add another layer of 'tufa.

You can always add 'tufa to 'tufa as long as you mist and use slurry. The drywall tape is to keep the crack from traveling up to the next layer.

Deb

You might have a structural failure on your hands. But it can be fixed I think. Let it harden well before demolding. You'll need to reinforce the shell before you put on another layer of 'tufa. Hardware cloth comes to mind but there could be many alternatives. Maybe just wrapping some heavy gauge galvinized wire around a few times might be sufficient, anything to hold that crack so there is no movement that could reopen it.