OP, frosting is pitting, just not deep and it comes from corrosive primed ammo or ammunition loaded with Cordite type propellants to name a few.
Deep pitting comes from not cleaning properly after shooting such ammo and can happen even with modern non corrosive ammunition when proper cleaning and storage isn't done. I've seen firearms with dead insects in the bores and pitting shows up where the bodies were.
I've seen some specimens in pretty rough shape, including a couple of garage finds that were stored wrapped in blankets, on the rafters since WWI. They were brought back to very decent condition.
First, I would take them down completely, make sure the bedding isn't soaked in oil/crud etc and check for pitting on the metal in touch with the wood.
If you find anything you don't like, fix it. The wood will clean up very nicely with detergents such as liquid dish washing soap. Don't sand it down. Maybe steam out some of the dents. Clean the checkering with hard gunsmith type tooth brushes, usually available at your local gun shop for a buck. Use the same detergent when using the brush. This provides a bit of lubricant, to keep edges sharp and it is also a very mild degreaser.
If you don't have a large container to hold enough Diesel, soak an old towel or paper towels, then wrap them in a green plastic garbage bag overnight. This will loosen most of the crud and make any pitting readily visible.
Once this is done, time to go after the bore.
You don't give a lot of information, other than there is some frosting. Have you cleaned it to ascertain it's frosting or dust???
Then there might be jacket fouling left behind as well. As mentioned, Wipe Out will remove most, if not all of it overnight.
Depending on the type of fouling it is , such as that from cupro nickel jackets, you may need to go to a stronger solvent such as Sweets 303. Be careful with these solvents. They can do some serious damage to stocks.
It's not a difficult job but it can be tedious and time consuming.
If you're only worried about the bore, Wipe Out will likely do the job.