Paul,
Whatever finish you use, it should be one that dries quickly with little gas out during cure if you intend to deliver it shortly after coating and the client intends to fill it up with his stock of wine. Wines can be effected by the fumes given off by coatings. If your client has the patience to buy and store wine for any degree of time to improve its quality and value, tell him to hold off introducing the wine to the racks for while. That "while" would depend on the coating you eventually choose and the ventilation of the area its installed/stored.
Many wineries do not coat their wood racks for this reason.
Cheers,
Kent