I've been washing my muzzleloaders with hot soapy water for years. If you know how to do it right you get zero flash rust. Clean, dry, and then a good quality gun oil. I could shoot one till I had the rifling wore right off of it and still have no pitting or rust.
Of course you can do it with solvents with good results also but I find water is effective and very inexpensive with the powders I shoot ( real black and 777). I think solvents are necessary with BH209.