Pat: maybe this will help
Here is a video tutorial to show you how to remove the rust from your drain pipes. https://www.youtube.com/watch?v=3ilMucKA_G8
What kind of drain pipe are you asking about?
Hi Pat: You don't say what your pipes are made of. I'm going under the assumption that they are galvanized. Not easy, but you can do this. Here are some sites that should help:/
If the rust isn't too far down the drain, try this:
You have to scroll down a bit to get your answer here:
Hi Pat, I am not sure what your pipe is made of so I hope this helps you out
I'd try this mixture: https://www.youtube.com/watch?v=6x5BNcHyiiI
If these are sink drains, then it may be cost effective to replace with plastic pipes and P-Trap. Plastic doesn't rust and are inexpensive. Might be worth your time and costs of trying to fix old rusty pipes. Remove old drain and take to Home Depot to get the correct size and configuration. Then you are done you will have rust-free drains and a clean P-Trap.
Call your local Habitat for Humanity. They usually have volunteers that can help you or know people that will do it as a community service.
On the outside or inside of the pipes? Metal pipes aren't used that much, if at all, because of the breakdown (rust) which leads to all kinds of problems. Better to replace them. Good luck and stay safe!
If light rust, use WD-40. If medium rust, use sandpaper or a wire brush. If heavy rust, use a mask and a sanding machine. or replace the lot with plastic pipe. Best wishes........
Here's a video that might help - https://www.youtube.com/watch?v=XE_fwxuZ1Z4