I don't sand props to balance them nowadays.
I use clear spray lacquer on the back of the light blade (where it will have the least effect on aerodynamic imbalance).

For the hubs, I use small holes with Lead shot epoxied in.

I saw both those tips here on RCU a few years ago.

However on any engine I care about, I only use decent props (generally the more expensive brands) and if they are so far out of whack that they will need much more than 2 or 3 coats of lacquer or 3 or 4 pieces of shot - I don't use the prop. For "Top End" brands of props, I check them as soon as I get them (I use masking tape to hold on weights to get an idea of the balance required) and if they are badly out (it has only ever happened to me on one occassion), I send it back.

One other check I do is I put the prop on my balancer (home made setup) and check the blade tracking before I do anything else. If the blade track is out of whack, no matter what you do, that prop is not going to perform well.