If you have very small cracks or high porosity, you can apply a thin layer of Portland cement mixed with nothing but bonding agent and just a tiny amount of water to the inside of the bowl. A slurry of this mixture is extrmely dense but it does require very controlled curing. As soon as the surface gets it's initial set, mist it thoroughly and seal it up as air-tight as possible (plastic garbage bags work great). Then re-mist and close it up again daily for the next 4-5 days. Once the curing process is well established, you can let it sit for a few more days in the bag or plastic wrap, then uncover it to dry. Once the surface is thoroughly dry, apply several coats of a good concrete sealer. A water-based sealer will be more "bird-friendly" than some of the solvent-based products. The solvent-based products seal much better but are rumored to leach small amounts of toxins into standing water.

If you have larger, more open cracks, you can use the finishing technique above, but must seal and secure the cracks first with a commercial-grade "crack sealer" first. HD & Lowe's both carry what you need. Best of luck and enjoy the birdies.

hmmnnn...I've seen so many birdbaths with sealant on the inside, looking ugly. Reading through this thread I had a thought - for the bath section, why not use a shallow plastic basin, pack tufa on the inside, up over the edges, and down the outside? In other words, leave the plastic basin, invisible inside the fabric of the birdbath. Another alternative, if you were building the tufa up over a former, would be to lay on a good layer of tufa, then cut a sheet of plastic (thick garbage bag, something like that) into shape, a couple of inches smaller all round than the tufa bowl, then continue packing on tufa on top of that. You'd need to make sure that there was enough tufa at the edges to hold the sandwich together.

Attached link is to a rough diagram that might make things a bit clearer.

Here is a link that might be useful: {{gwi:65959}}