How to force to login after logging out using google provider

[Armando Noval]

Polymer keeps the last user logging info and doesn't ask for user and password 

    <firebase-auth

        id="pauth"

        app-name="NSC-ALPHA"

        provider="google"

        signed-in="{{signedIn}}"

        user="{{user}}">

    </firebase-auth>

to login i'm using this function

  signIn: function() {

        this.$.pauth.signInWithPopup();

      },

this  tries to display the login form, but as it has the user info loaded before it inmediatly closes and continues as it is was correctly login.