The best "gunk" remover I have ever seen is with electricity.
I have an article in the most recent issue of Rod & Kulture (issue #15) on removing rust with electrelissis. No, I cant spell it, but I do it and it works great. It also removes ALL gunk and paint. Its amazing.
You may be talking about a piece that is large, so short of creating a large pool to submerge your part in, it wont work for you. But if you have smaller parts that can fit in a 5 gallon bucket or a 35 or 55 gallon plastic barrel or garbage can, it will work great for you.
Lots of info on the web about this and its stupid simple. There is no toxic stuff left over. Just iron rich water with baking soda in it. You can dump it on your lawn.
Again, all the rust, gunk and paint, gone with no work. The cool thing too is that it will convert some of the iron oxide (rust) back to iron. The converted iron it stable and can be painted over no problem.
Thats my $.02