Therein is your problem. Weldbond re-hydrates outside. Simple as that. Same thing happened to me in my early days of mosaicing. Only disaster I've had, exactly as you described. So sorry. That's why I keep saying DON'T USE WELDBOND FOR OUTSIDE PROJECTS - EVEN TO SEAL.

Oh man! I hate it when things fall apart. the only thing I can think of is that the thinset was not compatable with the flowerpot material. If there was a very smooth finish on the pot that also may cause the thinset to pop off. So roughing up the surface may help or you could try a primer. I'll bet another type of adhesive will do the trick. I think weldbond or silicone will stick to fiberglass. I wish you luck. Don't throw in the towel. I find that a cup of tea makes everything better!...or a glass of wine.

ahhhh Slow.. I remember you saying that you had a problem using weldbond outside, which is why I mentioned it, but I'm so braindead I can't be positive I DID use that, sure hope so, don't want a problem like that. There is still one teeny piece left on both pots, debating on cleaning them up and slapping more thinset on. or just give up on it totally..
You do make me feel better knowing you had a similar problem. Thanks , I was extremely annoyed with myself,

I will never used weldbond outside again.. oh no.. I sealed my yellow brick road clay pot for Sir Wiggle. Screaming.. Oh well.. If it falls apart too, you'll hear me screaming. We'll call it a science experiement. I'll post it as soon as I get my camera set up again...

I think it's because of the fiberglass substrate. The thinset had nothing to grab onto, I believe it needs a porous surface. I think you'd be better off with silicone as an adhesive in this case. As for attempting to seal the pot with weldbond, I'm not sure why you would've needed to as the fiberglass pot wouldn't have been porous, nothing to seal. I'm sorry this happened, it's frustrating when something like this happens!

Thanks Gin and Eggshell,

I wish I had the original labels on what those pots were made of, I just know they aren't made of clay. I tend to err on the side of caution, and figured, might as well seal.. Sigh..

Not sure what I'll do with my pots, but I'll sleep on it and tomorrow should be a better day...
thanks again, Slow, Eggshell and Gin, appreciate all your thoughts

Mer- you don't have to seal fiberglass..it is a structure unto itself. Only seal terracotta or concrete and on the inside only, the outside will be sealed with thinset.
As Slow told you, Weldbond is the culprit...DON'T use it OUTSIDE! When it gets a little bit of moisture, it will rehydrate as Slow told you and when it rehydrates, wammo..
the tiles come off. They are being held together by the thinset. I know that the fiberglass pot will not get wet from the inside, so why seal? Seal your finished work with Tilelab sealer after it is all done and you are set.
Put your Weldbond up high so you can't reach it, you don't need it outside..

Totally off the subject...your dog is beautiful. What kind of dog is he or she? Adorable.

Thanks Flag for the tip. I used the last of my bottle of weldbond yesterday. I still love that stuff cuz it has no odor. I'll have to make myself a sign by the back door, no weldbond allowed through door!! LOL

Still debating on how to fix this, do you all think I could scrub it down and use thinset ?? or just chalk it up to a lesson learned??

Tilly, that's Rascal, my aussie. Isn't he adorable? you should see his sister, the beauty queen.

Mwe- I would be inclined to scrub it down and adhere what you have with thinset. I think you will be surprised how good it will look.

Believe me, MER: If you could see FLAG's back yard, you'd know she knows what she's talking about. Follow her advice. I'd say also to use some hot, soapy water to scrub that wicked Weldbond off and begin again w/thinset. What you really did was glue your tesserae to the Weldbond - not the fiberglass substrate, so when the glue rehydrated, your tess fell off. Took me awhile to figure out that's what happened to my pot. Makes sense doesn't it?

Don't give up my dear! We all have those horror stories, but look at what these gals are doing now!
We will grow up to be mosaic queens too!

Slow and Flag, I have seen the photo's of both of your places, and believe you me, I trust you know what you are doing.

I will scrub those dump pots and try again with thinset. Second try and a bit wiser.. Thanks.

Texas, no, I won't give up, it is way too much fun, I was just so frustrated to see those big chunks falling off. and yes, we are going to be the next batch of mosaic queens. ROFL