Most, if not all, die hard helmet collectors will tell you not to do anything to them. When you do try and clean the helmet, coat it, etc it then becomes a messed with helmet. Thus taking away the originality, and lowering the value of the helmet.
Keep them in-doors, in a dry, low humidity place to keep them from rusting. Mine are sitting on a shelf on display in the living room of all places. Wife and Kids did that for me, gotta love them!
I have a WWII American M-1 helmet, Viet-Nam era M-1 helmet issued to me in 1986, Bristish WWII helmet and another one of that style that I can't identify yet. My Kevlar helmet used during Desert Storm, another one I used during Bosnia, another one I used during Afghanistan, and my last Kevlar I used State-side for exercises. WWII German helmet, plus a collection of flight helmets, and couple of Communist helmets.
Here's the correct list, found it on the hard-drive, it has more information and corrected a few typos on my part. This list is an ever growing list. Feel free to use it, and add to it.
Known steel suppliers of the WWI British helmets:
FS = T. Firth & Sons Ltd Sept 1916 – 1918
BS = W. Beardmore & Co Ltd Jan. 1916 – 1919
HS = Hadfield Ltd Jan. 1916 – 1919
V = Vickers Ltd 1916 – 1917
MS = Miris Steel Co. Ltd 1916 – 1917
A = Edgar Allen and Co. Ltd 1916 – 1918
F = Thomas Firth and Sons 1916 – 1918
O = Samuel Osborne & Co Ltd. 1916 – 1918
B = Bury's & Co. 1916 – 1918
Known makers of the WWI British helmets:
D = James Dixon & Sons Dec. 1915
HW = Hutton & Sons Dec. 1915
HH = Harrison Bros. & Howson Ltd. Dec. 1915
M = J&J Maxfield & sons Dec. 1915
R = John Round & Sons 1916
V = W&E Viener Dec. 1915
Known makers of the WWII British helmets:
BMB = Briggs Motor Bodies Ltd of Dagenham
F&L = Fisher & Ludlow Ltd of Birmingham
GSW DP & H = General Steel Wares Department of Pensions & Health
HELMETS LTD = Helmets Ltd of Weathhampstead
RO&Co = Rubery Owen Co Ltd of Leeds (Spelled as RO CO or RO&CO)
WD = William Dobson & Son of Birmingham
= Patent Pulp Manufacturing Co Ltd
JSS = Joseph Sankey & Sons
Known makers of the WWII British liners:
VERO II
TTC 2
FFC II
FFL II
V.M.C II
"N" LTD II
HELMETS Ltd I (Helmets Ltd of Weathhampstead)
Known makers of the WWII Canadian helmets:
CL/C = Canadian Motor Lamp Co
AG/C = Aluminium Goods Co
GSW = General Steel Wares
Known makers of the WWII Canadian liners:
Baskstay Standard Co (Also made chinstraps)
Viceroy Manufacturing Co = C/Arrow or VMC
Known makers of WWI American helemts:
ZC
ZF
ZJ
Mike
USAF Retired