The best way to avoid working on pitting: Don’t buy a pitted gun or make sure the pitting is superficial! Pitting is a very localized form of galvanic corrosion that leads to the creation of holes in the metal. It will occur when the composition of metal is not uniform and the process is accelerated in presence of chlorides (Cl-) and thiosulfates (S2O3 2-). Black powder is a mixture of sulfur, charcoal, and potassium nitrate (saltpeter); the sulfur and charcoal acting as fuels, while the saltpeter works as an oxidizer. Black powder residue consists of potassium carbonate, potassium sulfate, potassium sulfide, potassium thiosulfate, potassium thiocynate, carbon, and sulfur. All the potassium compounds are salts and considered corrosive. The sulfides in the emissions create the rotten egg smell we all disdain. To the right you see an electron microscopic image of pitting.

Areas of deep pitting can be cured by welding a layer of steel on the affected area and machining it in the original shape. This is a time consuming task that requires much professional knowledge and experience.
If the pitting is not too deep one can choose to grind or sand the surface till it is smooth. Care must be taken to keep the original looks of the gun intact and spare engravings and punches. It’s a tedious job that will take many hours but the end result can be very satisfying. I choose to do this only if it is unavoidable and use waterproof sandpaper (P500 ... P1500) that I support with pieces of wood in the desired shape.

The picture on the left shows a cylinder with pitting that can’t be restored without destroying engravings and proof marks.