I use normal washing up liquid for this. I just put it into hottish water, the worse the rust the more detergent I use. Leave it to soak for a while, then I clean the pony in the same water/detergent mix it soaked in, then rinse with clean water.
If the tail is really bad, I use neat washing liquid on it - just put the tail, rusty end down, into a cup, then pour in the liquid until the whole rusted part is covered. Again, leave it in for a while, then take out the tail, drop a little water on it, and lather up ^^ Then rinse away.
I never use metal washers on custom ponies or on ponies I've had to clean. I tie the tails in with wool, or those plastic tie tags if I have any around.