acid is bad for metal pipes. So is liquid plumber. Proper tool is snaking the drain.
I have a 40 year old house and one of the lower drain pipes had to be replaced. It was sort of a bad design and just held water and crap in it anyway...but it totally rotted along it's lower line.
Plumber said they probably had been trying to fix this problem for years with acid type products.
We re-engineered it in place and it should drain down now. The plumber said the original dude working on it ran out of so and so and so and so then just proceeded to make due with what he had apparently. In the end he basically created a drain trap below the house for the kitchen and laundry (thank god no toilets up stream).