my cloudsql instance is refusing connections from my gae standard environment app

Question

GAE standard environment apps cannot connect to CloudSQL

1. I have one project that includes an mysql instance which works.
2. I can access it from an app in that project, but not from other projects in the organization.
3. I've enabled CloudSQL Client in the other projects, have included their service account in the mysql hosting project and attempt to connect via the cloudsql connection string, but it refuses the connection. I don't know how to fix this. I've done this in other accounts but this is a new one.