I have had some time to work (okay play) on this helmet and I've managed to get most of the cleaning accomplished. As you can see I did not get too invasive with the cleaning. I started with the Wappen which was caked in dirt and rusted on the reverse. I soaked it in water with a mild detergent and that floated off most of the grease residue. I used a soft toothbrush on the reverse only to remove dirt and some of the rust. You can see there is very little difference.
BEFORE:
AFTER:

I did the same with the Kugel, with the same minimal results. Again I did not use a brush, only a soft cloth. As mentioned earlier I did not touch the Kokarden or the chinstrap at all.
I then set out to clean the shell. I had let it dry out for a day after removing the Wappen to ensure that it was completely dry. I then used a soft cloth damp with water only and started the task. I again had some discovery learning when I found out that the front visor and the rear neck guard were made of fiber and not leather. The ear pieces are also made of fiber and one is not cut correctly.
LOOK AT THE FRONT OF THE EARPIECE:
With the damp cloth I managed to remove all of the sticky stuff on the shell, so I got my set of soft paint brushes and Q-tips and began cleaning around the stitches and seams. This went pretty well, causing no damage. The helmet shell behind the Wappen was in bad repair and some of the leather (it might actually be fiber too) was worn and raw. I used a dab of Kiwi shoe polish in these areas and around the holes that are worn by the grommets primarily to seal against future moisture. I then took a soft shoe brush and began buffing out the helmet shell, applying no pressure. I am very pleased with the results:
As you can see I did not try to make it shiny and new, just clean and presentable.
I was asked a question about the issue marks on the rear neck guard, and I can now read them. The marks are "BJA XI w" and next to that is an "f" or possibly an upside down "J". As near as I can figure these marks would belong to "Bekleidungsamt XI Armee-Korps (Cassel), J for Instandsetzungsamt (Repair Depot)". I will try to research these marks further.
I have no idea what the "w" would be. BKA XI would have handled FAR 11, 18, 19, 47, and 55. I can find no Fuss Artillery that came under BKA XI. I found a sample of this issue mark on Colonel J's website "BJA XI w" and it is possible the the upside down "J" is actually what is left of a "ga", but I don't know what that indicates.
So far so good. The next post should show the completed project.
John ](*,)