Patrick,
About four years ago, I prepared half a dozen polished samples of brass, to try to establish what would be the best product for protecting the shine on my models.
I then gave them the coatings, from both spray cans and paint on products.
These were left in an open box on the top shelf of my shop, and after a year, all showed various tarnishes or breakdown of the coating, all except one.
It still gets the odd check now and again, and still hasn’t shown any deterioration, except for a few scratch marks where it has been kicked about a bit.
What was this wonder product?
Because I had it in my shop for protecting wax lettering I used on my model boats, it was in fact just tried for the sake of it being there.
It is Letrset 101, a fixative and protector for what I have already mentioned, and was put on as a superfine coat sprayed from arms length above the polished plate, just how you would normally do it with wax lettering.
I used it on a couple of my engines that went to other continents, so I can’t check it out to see if it is still working, but from the sample I have in my shop, I don’t doubt that it still is.
Bogs