Be careful with flux, excessive amounts can make a bad joint.
I did a theater for Georgetown University a few years back, we had a subcontractor soldering whirlwind mass connectors (between 48 and 120 pins per connector), he kept bragging about being a "mil spec" solderer, and told us that you HAD to add flux to make a proper connection, even when using flux core solder. After soldering all of the connectors on site (around 60 of them), we began testing the system, at which point we found that around 95% of his solder joints were over fluxed and would not pass signal.