They sandblast brick houses to remove paint. One of those plastic bead blasting companies that strip classic cars might be able to do your birdbath if you brought it in to them - not totally sure they do concrete but it would be worth a phone call. I know around here that type of blasting is not expensive. Our local handyman show interviewed someone who did that and he would strip a metal mailbox for 5 dollars or so. Chemical strippers would be too toxic for birds, in my opinion