As alanw noted, you will clear C&I at Seattle.

You will be arriving at the South Terminal (S Gates) and your flight to SFO will be departing from the North Terminal (N Gates) at the other end of the airport.

If you are a UA 1K or 1P, you can use the elite security line for the North Checkpoint, otherwise you might be better off re-clearing security at the South Checkpoint and take the shuttle tram from Main Terminal South Station to Main Terminal North Station and connect to the tram to the North Terminal.

The North Checkpoint can get very busy, but not a lot of airlines use the South one (and most that do are international with late night flights), so I find it can be faster.

SEA Map - http://www.portseattle.org/about/maps/transit.shtml