My bird bath has a crack in the basin where it holds water(or did do) has anyone used a poxy resin to seal the crack and if so what manufacture did you use
It might stop the leak short term but it won't last long. The best way to use resin would be with fibreglass matting. You should be able to get a kit as used for car body repairs and make a full liner of the bath. I can give you instructions if you want to go ahead.
I would be temped to stabilise the crack by securing it with a steel bracket and then paint the inside of the bowl with a waterproof paint, obviously one that isn't toxic to birds.