Since every leg is a little different and the tail section isn't perfectly symmetrical, I have to basically custom fit each leg to its section of the fuselage
So I'm just using a tiny piece of sandpaper and the tip of my finger. Making them out of softwood is coming in handy, as removing material is pretty easy.
I also found that rubbing the leg up and down while applying pressure makes slightly shiny spots that I can then sand.
It's a laborious but effective process
The sandpaper isn't flexible enough for that, it doesn't like to curve in two directions at once
You can soak sandpaper in warm water to make it more pliable.
I recommend trying out out with a small piece first, to get an estimate on how it holds up.