With four-plus decades of making cartridge belts under my belt, I have not found anything that works permanently to prevent verdigris from forming on cartridges if you leave the rounds in the loops more than a day or so. This is due to the friction generated on any treatment after inserting and removing the cartridges from the loops. The best thing is to unload the belts after every shoot, unless you are going out again the next day. And, even then, if you are in a wet climate, I'd still recommend removing the rounds from the loops.