I have no practical advice to give, but the model is very interesting. Is this the old Athearn kit when it was metal rather than plastic? You do not mention a scale.
I think you’re going to have to experiment some, so I’d try some white vinegar in a small area and see if that works or maybe that inexpensive jewelery cleaner. Another possibility is lime away, but that stuff is strong and may stain more than clean. Good luck, Mike.
Just in case you didn’t know, that flat car that goes with it is referred to a “boom car”. Knowing that should make it a little easier to track one down.
Keep us posted on your progress. It should make a nice looking crane once it is painted and all that jazz.
I bought an old Athearn all zinc metal crane at a swap meet years ago. After assembly I cleaned it by spraying with Formula 409 and and water before painting. After painting I strung up the sheaves using high quality fishing line. It is one of my favorite models I own, and far nicer than the plastic one sold nowadays.
I’ve got this one in my collection. It’s over 50 years old (but, so am I) and I got it from a yard sale “box-o’-trains” when I was a teenager. I can’t find any identifying marks on it.
It is all metal, and weighs as much as a locomotive. The trucks are 3-axle. (One is broken, so that’s my first repair on it.) The wheels are plastic, but I may have replaced them myself when I was a kid. The boom and hook are both operated on pulley systems, which are strung with heavy black thread. The winches are turned with a metal key inserted through a hole in the side of the crane, which can be seen in the picture. (Unfortunately, I’ve lost the key, so that’s something else I’ll have to fabricate.)
