I'm probably no expert for sure... but just take some off the heavy side like your doing... but there is another trick. You can take a drop of CA and spread it instead of sanding. Works for me. Also if you get it where it's "level" across. Then the force of imbalance is pretty much minimized. Think about it. How much centrifigal force could the Chord actually produce? But if you're really worried put a drop of glue on the heavy side at the center. What kind of prop is that? Is it painted?