My bathroom sink and tub go into the same horizontal iron drain pipe across the basement ceiling. That pipe apparently doesn't have enough angle to it, so all sorts of crud collects in it. My first week in this house, the tub backed up and I had to open the trap for that pipe and snake it out, which was NOT something I want to repeat often. The previous owners had not taken proper precautions upstairs. For instance, the tub drain's screen was decorative, but otherwise useless in terms of stopping hair, so it's been replaced.
Meanwhile, it seems things are beginning to drain slowly again. Replacing the basement pipes is NOT AN OPTION at this time. And, plunging doesn't work because the sink & tub share the drain, and there's no way to maintain a seal for the plunger to work.
The usual Mr Plumber stuff seems like an expensive way to maintain the pipes because the distance from sink/tub to the horizontal pipe is about 10 feet. Would sulfuric acid be appropriate, if I'm positive that I still have at least some drainage and can flush the stuff out thoroughly? By the way, the whole system is metal pipe. Chrome-plated stuff under the sink, iron in the basement.