Hi! I'm new here...

On our return trip we are spending three days in Singapore. When we depart SIN for SFO on UA 2, we are flying to SFO also in Polaris. What lounges in SIN can we access prior to the flight? Can we access any of the SingaporeAir lounges? (My husband is platinum on united, so star alliance gold, and we are also both AMEX platinum cardholders). Based on what we can access, what lounge would be the recommended one?

Thank you in advance!