Dear Dan & Blanche,
The coating I have used that does what you describe may be Syncrolac
Y278 which is an acrelyc seal. The other is Durachem Clear Y291 which
is an epoxy. The are air dried or baked on. They only need a fine
layer to seal the object. This material will last for many years.
Their literature says it works great on copper and brass. I used it on
I got my supplies from Seagrave Coating Corp. in Carlstadt NJ. This
was back in 1991. Their phone # is 201-933-1000. They have some
technial info on the products they will send you. This is the best
clear sealant you will find.
TR the Teacher & Student