the only time i see rust is when i use hot water to clean.i used water and hoppes bore cleaner to clean barrel for 35years.i bet if i had just put in the bore butter without that bore cleaner , i would have had rust.dont know,but hot water causes a lot of rust to come out on the patch.now, i use sheath and no more bore butter even tho it worked for me all those years.only time i am using bore butter now is for patch lube as i am getting back into roundballs and i gave up maxi-balls for powerbelts now.