Frosting is a term that is becoming much more common to describe bore condition . Frosting to me brings up an image of white? I have had a lot of antique guns with dark in the grooves. All can be cleaned somewhat over time with soaking bore with Kroil , Gunzilla or other bore solvents,   brushing , shooting jacketed ,etc. Some times this dark gunk or rust is obscuring corrosion ranging from from slight pitting to big pot holes. Is the newly visible corrosion the "frosting"  ?

Phil

When frosting is used to describe a bore, I think of a flat surface with no shine. Like flat black paint.  Frosting comes in many colors, on cake.   

      Joe. 

Eeeuuww !

Those Parkenfarker gunsmiths are far too common. An online course an Atlas lathe and I are a gunsmith. I still have a memory of a Wesson 2 trigger at a gun show. It had been buffed on a wheel till there wasn’t a straight line anywhere Hot blued and nickel plated at the same bumper shop that did the buffing. The stock was sanded down till all the metal stood about a thumbnail proud. Oh and it had a $4000 price tag on it. When I asked why so much I was told its museum grade! I still have nightmares about that gun.

40 Rod

Then there is the Parkenfarker wheel. Often used to clean bullet molds seen on fleabay.  
Parkenfarker soap is used in frosted bores to make them look shiny new. 

     Joe.