There are 2 products that will remove every bit of rust, no matter how bad it is or how deeply pitted it is. I've used both for years on items that had 1/4 inch solid rust all over them. These 2 products will leave every bit of the metal completely rust free.
First is Phosphoric acid. It is sold by automobile paint stores that sell Dupont automotive paints, under some brand name. It is sold in 2 strengths, one for removing serious rust, and one for removing light rust. The difference between the 2 is the concentration of Phosphoric acid in the bottle. It will state this on the bottle. The weaker one is used mainly for wiping on a LIGHTLY rusted, paint-less steel car body, and you are supposed to let it dry on the body. The more concentrated one was for parts immersion. Either will work as a parts immersion rust remover, but the stronger one works faster. You fill a plastic bucket with it, immerse the part in it, and cover the bucket with a rag. Let it sit overnight in a cool place, and next day look at your part. If the rust isn't gone, put it back in the bucket. Phosphoric acid burns your skin but does not eat through your skin, unless you were to leave it on for a long time. Wear rubber gloves! If you splash it on your clothes, holes will appear next time you wash them. Wear eye protection! Have fresh water nearby, in case you need to wash it off your skin. It stinks bad, so do it outside, in the shade. Phosphoric acid will attack non-ferrous metals: copper, brass, some plated items....don't use it on brazed parts. Phosphoric acid also affects surface temper...don't use it on springs, files, etc.
Next is a product called," Evapo-Rust".
www.evapo-rust.com This product does everything Phosphoric acid does, but much more slowly. But it does work just as well, and is MUCH more safe. It doesn't stink or burn you skin. It also has the advantage of not harming non-ferrous metals and chrome. Once I tried it, I never went back to Phosphoric acid. It also is an immersion product and the rusted item must be totally immersed for as much time as it takes for the rust to dissolve.
The problem of how to immerse an exhaust pipe is solved by buying a length of the correct diameter PCV pipe, and gluing a PVC cap on it. You then fill the pipe with Evapo-Rust or Phosphoric Acid, and let it soak until the rust is gone. Then turn the curved part of the exhaust pipe that didn't fit in the PVC pipe around, and immerse it in a bucket of the same solution. Always cover the bucket mouth with a large rag, to prevent evaporation of the product, and leave it in a cool place.
Once the piece is rust free, it will quickly rust again so be prepared to coat it with something.
Either product can be re-used again and again. After you remove the rust from the piece, place a large plastic funnel in the mouth of the original container. Place a cotton cloth about the thickness of a t-shirt down into the funnel and over its sides. Pour the used product into the funnel slowly, and the clean product will go back in the bottle while rust will get caught in the cloth.
Naval Jelly is nothing more than a VERY weak solution of Phosphoric acid mixed in some medium, and is for amateurs. Wipe-on solutions will not remove heavy rust.
I have often bought heavily rusted screwdrivers, wrenches, sockets, etc., at garage sales, swap meets and flea markets for the price of scrap, and soaked them in Phosphoric acid or Evapo-rust. After the rust was completely removed, I rinsed them off, lightly oiled them and used them for years. You can build up a nice collection of tools this way, dirt cheap.