You could also have sealed the basin of the birdbath by adding a thin layer of mortar mix. You would clean the birdbath, wet the birdbath with water so the hypertufa is damp, not wet, to the touch and use a mortar mix that you add acrylic fortifier to the water. Less steps to seal and if you made the hypertufa you already had portland cement. Billie

Hi Birdlover2010, just read your blog on Sealing a Hypertufa Birdbath. I will try this because I want to preserve the look of the hypertufa and not cover it with mortar mix coat. How has this thick coating of Behr Concrete and Masonry Waterproofer held up since you did this in 2010? Would you recommend, now, a different application? I purchased this Behr product this week.. and want to try it as you describe.. but was wondering how it has held up. I hear that Thoroseal is an alternative product (I think it comes in white not sure about clear). thanks

Behr Concrete and Masonry Waterproof - how does this impact the water and the birds, WARNING! This product contains chemicals known to the state of California to cause cancer

and birth defects or other reproductive harm. From their web site,

I made many birdbaths and sold most of them. I was always afraid to use any commercial sealer for fear of poisoning the birds. I just doesn't seem like a good idea. I never had trouble with leaks, but I made the bottoms of the basin part a good two inches thick. I would think coating the bottom with some mortar mix would work.