I would not advise turpentine or linseed, both of which will stink up your customs officer helme for another 100 yrs. I do refinish ie fill in spots where the lacquer has flaked off. However, This is an involved process which I am not going to get into here. In the case of this helme I would leave it as is. You could sand down the worst of the blobs/bubbles and then use a cotton ball dipped in lacquer thinner to smooth things out. you will still have some bumps in the finish as you would have to sand it right down to get things absolutely perfect. Next apply multiple coats of black shoe polish to fill in and smooth. So now hopefully, (if you don't screw up) you have a semi smooth shiny front visor which does not match the rest of the finish on the helme. At the very least, you would have to shine up the rest of the helme with polish or maybe do the back visor as well?? Bottom line is if you do this process it is going to be pretty obvious that the finish has been messed with. You really have to know what you are doing and have access to a "finish substitue" as well. The finish on the older helmets in my experience is more tar like. You have perhaps read about the Napoleonic "tar bucket shakos"? One of the ingredients in this water proofing finish was something called "ashphaltum". I searched for the old finish formula years ago and found one in a reprint of old 20thC handyman recipes. This stuff was used on all sorts of leather harness for example. It had to be flexible as well as water proof. I found it impossible to replicate this formula because the chemicals listed in the "mix" were not longer available in stores....where the hell would you find ashphaltum??
Anyway, in my opinion, with helmets in this condition, it is better to leave this as is. Maybe apply some shoe polish to shine things up but nothing more. My two cents.
My default option is almost always 'leave as is' ... Which is exactly what I plan to do. I just wanted to find out if there might have been an 'easy' solution to a fairly common finish issue. Cheers!
