Traditional "plumber's putty" might do what you want. (The stuff normally used to seal between faucets and similar fixtures and the countertop)
Thanks! I have some. I didn't want to make a mess that's hard to clean up, but can't see where it would damage plaster. I just want to blank off around some jets or light fixture to see which individual one is the culprit. Leak is so slow, I don't know if dye would find. Someone showed some putty for that here that was used in the leak detection industry, just can't find. I've got leak narrowed down to spa, determined a leak rate over days of spa, and just need to test against that. My PB is going to fix, but it's going to be after Thanksgiving, next week, before we speak again, and I want to make a better informed reply back when time comes.