A little more than a week ago I found a small ditch with two German helmets. They did look a bit rusty but I knew there would be some original color left under the rust and muck. First I gave them a wash with soap, warm water and a brush. Then I prepared a very hot bath with added oxalic acid for the two lids. I used somewhere between 50-100 gram acid for ca 20 litre water. I submerged the helmets and after 30 minutes I took the up and brushed them under fresh running water. Now lots of color were showing, but they needed a second go in the acid bath. Half an hour later I repeated the rinsing process but decided that a third go in the still warm acid bath would be to prefer. When I was done with the third brushing I placed the lids into a bucket with fresh clean water to neutralize and acid left on them, and later in the evening I could dry ther lids and give them a cover of the microcrystalline preservation wax I am making myself. The next morning the wax had dried and I gave both the lids a polishing with a rag before I took a few pics of them in the daylight. I am very happy with the result as both the helmets came out pretty good looking.
Directly out of the ditch.
After washing them with soap and water.
The result after the oxalic acid bath and wax treatment.
Video from finding the helmets below: