As Kutter has stated, there was some contamination possibly between the ribs. You didn't mention what type of rusting agent you were using and that would help. I use Laurel Mountain Forge's Browning Solution. It has it's own cleaner mixed in and for me has worked great. I use distilled water for my rustings and also in my etchings.
Most barrels do have some voids between the bottom rib, to check on this I use a .001-,002 feeler gage and go along the bottom rib to check. Sometimes it pays to boil first to get the contaiminants out, then clean thoroughly or in my case just start the rusting with the LMF rusting agent.
