I just bought and used a bottle of Liquid Plumber but I was just worried about whether the stuff might be able to corrode the rubbery seal under and around the top of the bathtub drain. I live on a second floor apartment and the walls of my shower are already corroded enough to leak below, but that was patched. I meanwhile have a lingering phobia of leaks and was just wondering if I should be concerned about this seal. I can visibly see the seal. My apartment--a condo which I "own"--is about 18 years old, as is this bathtub.
I think the answer is going to be "it depends on what that rubbery seal is actually made of" -- here's the quote from the Liquid Plumbr FAQ --
However, we do not recommend using clog removers in rubber pipes. The products may damage rubber.
Is the part with the rubber seal removable? If so, remove it before you use the liquid plumbr.