I used to get after-rust problems w/Laurel Mt when I first started using it. Couldn't get it to stop rusting.
I think I was stretching out the use of the water in the tank. Residual chemical in it may have been enough to cause it even after carding.
Oil alone won't stop acidic chemicals fom rusting steel and L/M is acidic on the Ph scale though no acid in it.
I stopped being cheap w/the water or at least using a freash tank for the last boil. Seemed to help stop the problem.
Using ammonia or baking soda as a back-up certainly won't hurt anything and it's good insurance against seeing a scarry looking barrel after some unattended storage.
Use either @ cold/room temp.
I've gotten into a habit of scrubbing down the bbl or parts w/a kitchen cleaner like 409 and a simple scrub brush after the last coat and things are cooled off.
I figured there's enough alkali in those to do the job if it's still needed and the wet brushing does a nice final carding of the blue.
Room temp water w/the cleaner,,then rinse w/hot. The bbl/parts will then dry easily.
The laundry tub is a good place to do this.
