Pickling nickle silver and brass

I teach Jewelry Design to high school students. In class we use a
lot of brass, nugold, and nickel silver. I have never have used it
much in my own work.

To clean up from soldering we use nickel pickle. The problem is that
after multiple soldering, the metal is pitted, and has layers etched
away. The students are really frustrated getting the metal cleaned
up. Is the solution too strong, are we waiting to long, is somebody
ruining the pickle with steel tweezers? We will save a lot of time
and money if we get this worked out.

My other question is that if you have mixed metals, say, copper and
nugold, do you pickle in regular pickle, nickle pickle, or both?