Bal green star is at the cheaper end of the Bal range, but they do recommend its use on existing tiles. So yes it will be ok.
In a shower area I'd go for Blue or White star myself though.
I'm gonna say this once guys then refer to this post in the future...
Tiling on tiles is fine so long as your existing tiles are properly fixed and you don't overload the substrate. The likes of Bal specify in their product data sheets that their products are suitable to be used over existing tiles, so if they advocate the practice, who am I to argue? Ok, I'll allow that given the option, I'd prefer to remove the existing tiles, but this is not always a viable option to some.
Just reiterate.... tiling on top of existing tiles is not the Devil's work.
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