I would try just rubbing in some light oil with 0000 steel wool. It could be very fine pitting on the surface that gets invisable with an oil rag. The heat from barrel may be drying out the oil and showing the rust again. You may want to try different oil. Good ol'e 3 in 1 worked good for me the past 40 or so years. Some of the new super lubes you pay an arm in a leg for just ain't all that IMO.