Bad juju! I've never cleaned cork on a flyrod. Only time I've ever cleaned cork on a gear rod was after using bait. And even then it was a dunking in the river and using my hand to get residue off. I call it a patina on the handle.
Leave handle as is. Keep its soul intact. Next thing you are gonna ask is best way to clean your vest! Blasphemy!
Actually, I remember the saddest (and happiest) day with rods. My Uncle gave my my Grandpas old cane rod with old skeleton framed pflueger. Still has an odd looking flyline on it. Well, my uncle (not being a fisherman) said the grip was grimy, so he sanded it down. UGH!!! Had a beautiful layer over that cork. Remember it fondly. Looks nice and clean now, but would've rather had the grimy cork.
Funny! The last guy I saw "fishing" with plastic over his cork I was a bit suspicious. That suspicion was realized later as I watched him pull out his citation book and issue about a dozen tickets in a single hole to a bunch of gear guys that he cited for snagging, illegal rigs and fishing without licenses (not all the same of course). When he passed by my friend and I swinging flies above that hole I chatted with him briefly. He checked my license, hooked his shirt with my barb crimped fly and told me he wished there would be more fly anglers around. I asked for his card and he handed one over. He was a NOAA enforcement officer if I remember correctly. The hole below us cleared out pretty fast!
I told him that the cork covered in plastic blew his otherwise perfect disguise. He also showed me his hook, which was clipped off on his corkie and yarn rig. He said he planned to return the rod and did not want to get it dirty. I told him I planned on keeping mine and hoped to get it really dirty.
steal that can of scrubbing bubbles the little woman uses to get the soap scum off the porcelin tubs and sinks; spray it on, wait a couple of minutes and then scrub the grip with a soft brush then rinse with water; looks like new; old grips might require two applications-p-