A stained bathtub, or stained sink, from a drain cleaner can be repaired with a delimer. The drain cleaner stain will melt away.
The powerful drain cleaners, like Rooto’s Professional Drain Cleaner, contains sulfuric acid, i.e. battery acid. Using too much at a dose can cause Rooto and the crud in the drain to expand into your sink or tub. Trust me, I have been there, done that, and hated it. And it left stains in my white tub, which they warned of on the bottle.
Your problem is not new to us since we have sold Rooto Professional Drain cleaner since I was a kid. Back then it was the only strong drain cleaner worth using. Thank goodness, we now have drain-cleaning products that clean drain problems safer and easier. I prefer to save the use of Rooto Professional to the laundry tub and main drains.
Fortunately, another of Rooto’s products is the perfect cleaner for this situation, Rooto’s Toilet Delimer. Usually I recommend using this to clear the lime out of an older toilet giving it new life. It works very well for that and now we use it for the stains in the tub.
Before working on the bathtub stain, finish clearing the drain. Then run plenty of water through it to flush out any remaining drain cleaner in the pipes.
Open the bathroom window, or even put a fan there blowing the air outside. Wear rubber gloves and use a nylon brush to work the delimer over the stained area. The stains should melt away before your eyes without very much work.
Be careful not to be exposed to too much of the fumes caused by the delimer. Anyone not in tip top shape should hire this job out, do not take chances.