Tail rust is a common sight and it looks fairly nasty, sometimes it can stain the tail all the way down and washing it only makes it worse and spreads it further. You need to get to the source of the issue and remove it. This tutorial will also cover what i do to remove stink as well, its fairly basic and i use it to get rid of that "old smoke" smell they get from being in a home where they have been around smokers.
This is by no means the "right" way to do things, its just how ive gone about it. Please keep that in mind before you comment. The pony im using is a G3 Fluttershy Bait pony* (She was baited due to a dog chewing her foot.) But this works for ponys in great condition too, dont be afraid to fix them!
*Bait Pony = This is a pony that is beyond repair, or not worth repairing due to repairs being very noticable. People use Bait Ponys to make customs :D
Step 1: What you need!
Okay so basically you are gonna want to wash your pony, i have a tutorial on that already, just do all the way up to the part before boiling. Dry her just roughly with a towel and get ready for the operation!