Have you tried wax?

I would think a slurry of portland and (admix or fortifier can't remember which one) on the inside of the bowl to seal it and give it a nice smooth finish. I have only read about this so not sure of the final results.
Paws

Paws a slurry coat can work great but has to be applied properly or it can flake off.
The biggest problem is 'neat' recipes are horrible for cracking as they dry.
I would reccomend an exterior redimix like Quickwall or another mortar mix that has Superplasticizer already in the bag thus reducing the amount of water needed. This reduces the chance of cracking.

But I still think a few coats of Floor wax is the easier way to go.:)

Hi "magrit" and others,

When I use a hypertufa to make a birdbath, the easiest thing to make it water proof is to rub dry Portland cement on the birdbath when you unmold it. The dry cement is just sucked right into the hypertufa. If you wait until the piece is dry... just mist it and add the dry Portland cement. The appearance is different than hypertufa...but try it you may like it!! :-)

warmly,
Nancy, who loves birds and birdbaths!!

PS-my hypertufa mix is 1 part peat, 1 1/2 parts Portland cement, 1 1/2 parts sand & if weight is an issue, 1 1/2 parts perlite. I generally unmold after 24 hours.

Lowes and Home Depot carry a material called water blocking mix or hydraulic water block(or something to that effect)that is sold in the same area in the store where they sell quickcrete, portland cement, sand, etc. and that is made by the Quickcrete people. In my stores the material is sold in yellow tubs with a black lid if memory serves me correctly. This stuff is suppose to be used to stop water penetration through concrete block walls. The material mixes up as a black thick pastey material and it sets up very fast (personal experience talking). Perhaps this could be applied to the bottom of your bird bath to prevent the water from leaking through the structure. Applying it on the outside would prevent having to worry if it was leaching material into the water in the birdbath. Just a thought that I mentioned previously last year but never got around to trying.

Thanks everyone for the input. I hadn't thought of the wax, I will have to try that. This time though, I put a thin coat of portland/sand mix on the inside of the bowl. I just put some water in it this morning, and that seems to be doing the trick! Maybe it's the perlite that caused the leaking? I don't know for sure, but from now on I think when I'm using my perlite/crete mix, I'll do a thin portland/sand mix on the inside of the bowl right from the start.