Using CLR to remove buildup from cast iron drain pipes???

A few months ago, I had cut out my bathtub trap and replaced it with a tubular plastic one. The old one was cast iron, and about 50% blocked with a rusty-buildup on the bottomside of the pipe. I was wondering if anyone has tried using CLR to fix a slow cast iron drain, thinking that it might be able to remove the rusty buildup inside (house is 55 years old)?

I e-mailed Jelmar and they had this to say:

"CLR, Calcium, Lime and Rust is safe for use in copper plumbing and septic systems and does remove rust so pouring it down your drains could help if the cause of the slow drains was in fact a rust buildup. The effectiveness of CLR comes with contact. I’m not sure if pouring it down your drains would have the 2 minutes of contact that usually takes to remove a buildup."

Thanks.

I suppose it might work but in many cases, once the rust goes it leaves a hole behind.

If there's that much rust, the integrity of the pipe and fittings is in question...I think it's time for more mechanical work (remove and replace) rather than chemical.

IF CLR would work, which is not a given and also probably not likely, it could take YEARS to dissolve the buildup, NOT minutes.

This was the pic when I cut out the old pipe. I would imagine the rest of the pipes in the house look similar. Pipe integrity seems OK. Does this look like rust build-up? The gunk has sort-of the same texture as "astronaut ice cream" - dry and crumbly.

If it come loose, it was not rust. If you have to use a hammer and chisel, then it IS rust.