I've tried the polyurethane (Varathane) route and concluded I'm better off heavily treating handle by heavily soaking in a mixture of mineral oil and paraffin.
I prefer the feel of the unvarnished walnut and take care to only do a light washing of the handles. I'm leery of flaking and peeling of the Varathane finish, particularly at the front of the handle where it joins the blade.
I get enough exposure to plastic from plastisol jar lid liners.
Since I have a surplus of knives, I can afford to take one out of service to thoroughly soak its handles, wrap the wet handles in dry film wax paper and let the knife sit in a warm dry place for several weeks.