Sealing old smells in hemp rope.

How about modge podge?

I think I'd try soaking it in some bleach water (not too strong) to start with - that'd kill whatever mildew might be residing it in and prevent further growth (though it may lighten it and undo whatever grubby look you found appealing :-) ). Let it dry well in the sun. That may take care of the odor and you won't have to seal it. If it still needed sealing, I'd try soaking it in something like polyurethane (rather than a spray-on), so it'd get into the nooks and crannies. Lay it out in the sun (on waxed paper?) to dry. Good luck - I'd love to see what you do with it!!

rinse it over and over in fresh water till it runs clear, arrange in your own way and let air dry on a screen.

Here is an article that might help - https://www.hunker.com/13419920/how-to-remove-sisal-rope-odor

wash, wash, wash, and hang in the sun to dry until really really dry ...