Snomam, Unless you are planting aquatic plants, lining a planter, any planter, is not a good idea. How would you keep the planter from flooding during a rain storm or a snowy wintertime thaw. Planters in my climate zone need to be watered at least once a day during the Summer (sometimes twice) unless it's sedums or large planters with trees or shrubs. If you can't water every day and want a cheap irrigation system, take a bucket of water and sit it next to the planter, cut up an old towel in long strips, bury one end in the soil in the planter and put the other end of the towel in the water bucket. The towel will wick the water from the bucket to the soil. Unless you can hide the bucket it's not pretty. I use this method when I am away and can't water.

Thanks, billie_ann...to keep the planter from flooding, there are drainage holes in the bottom. They aren't left outside in the winter. I'm going to experiment with waterproof paint for basements and masonry.

One of the benefits of making hypertufa containers is that they hold moisture themselves which is good for the plants. I would not seal the inside.

Hypertufa by nature is porous, so I've found they don't hold moisture at all. I did seal the interiors with waterproof paint and it's working like a charm.