Most plastic props aren’t balanced. If you put them on a horizontal shaft, one blade will swing down. The spinning prop will vibrate the entire airplane, wasting energy.
Many people scrape the back of the heavy prop blade with a razor blade. This doesn’t work very well, because the straight edge of the razor does not fit the compound curvature of the prop blade. Also, scraping from side to side across the blade risks going over the edge and cutting your finger. I use a kitchen paring knife with a curved edge near the tip.
Angling the curved knife blade permits scraping off a wide area of the prop blade. Scraping toward the tip prevents hitting fingers.
Keep scraping the heavy blade, checking for balance from time to time, until the prop balances.
An even safer way to do this is to back the prop with a folded towel.
If you want to avoid the risk of letting out your precious claret, don’t scrape. Stick Sellotape across the tip of the lighter blade, building up layers and adjusting the last one until the prop balances.
Not as neat as scraping but easy and quick.
Really well explained, sometimes I find the props are so badly balanced I have resorted to using epoxy resin to add weight to the opposite blade.