Hope someone comes to your rescue - I can only tell you what I do. I w/never use anything but thinset on a concrete birtbath, even w/stained glass. If you want to paint the glass b/f putting it down on your thinset, I think that might work - I'd surely try it, if I were in your position. I w/NEVER use the premixed stuff - just don't trust it.

slow, painting is a great idea. but my husband was going to lowe's, so i went ahead and went with him and bought some white thinset. it's more expensive than the gray and it only came in 25# bag, but that's okay. i like lowe's thinset (mapei) ultraflex 2.

Great choice on the thinset. Listen to Slow, I tried the premixed stuff and have the disasters to show for it. Only Mapei for me too.

Hallelujah: Mapei Ultraflex II - my favorite. It has NEVER failed me on any substrate, except onto Duct Tape.

Yup, I just scored a whole 50lb bag of it for a buck...who throws away a whole bag?!@!! Happy Dance!lol! I just finished painting the glass on my pineapple(some parts)and it turned out great! What kind of birdbath is it? Cement?

actually i think it's terracotta - a stained terracotta. i started it last year and i may have posted pics. i know i did ask about my pieces falling off and y'all suggested i didn't have my thinset thick enough. so, i've now finished the base, the underside of the bowl and am working on the inside of the bowl now. i'm embarrassed that i left it sit for so long, undone. but it will hopefully be finished within the week. and yes, i'll post pics!!!

Flagtruck did you have problems using the premixed thinset on the columns you did, or was it something else?