AFAiK, none of the airlines has a single policy about club/lounge access that covers every airport club/lounge. If a club or lounge has heavy traffic from members at a particular airport (for instance, the AAdmiral's Club at DFW), they probably would not offer club access to a casual, one-time First Class passenger. If they are trying to build up traffic at a particular club, may or may not offer access to casual, one-time passengers. Also, some airports have both membership required Club/lounge facilities AND a First Class "in transit" lounge specifically designed for F/C passenger who are changing flights at that airport. You need to ask United about their facilities at SFO. (I'm just guessing that is the airport you have questions about.)
