If it's just the flash rust in the valley and crankcase (and exterior) spray the whole thing with WD40 and don't worry about it. The issue was probably where you kept it in addition to not getting it truly dry. A trick I've used in the past is to shoot brake cleaner on it and light it on fire. It burns clean away and will heat up the entire block to help drive the water out of the pores of the iron. You do need to "reapply" for a good 4-5 minutes until the block is hot to the touch. Then spray the whole deal with WD40 and let it dry.
Edit for those challenged by some things.. THIS SHOULD BE DONE OUT OF DOORS IN A WELL VENTILATED AREA THAT DOES NOT CONTAIN FLAMMABLE MATERIALS, WITH FIRE PROTECTION EQUIPMENT WITHIN EASY REACH, AND IF NEEDED WITH SUPERVISION OF A QUALIFIED INDIVIDUAL...lol
