How about Ambroids? That is what all of my Ultra Sports were built with. It is water proof, fuel proof and sandable.
Just use a Great Planes sanding bar to taper the control edges. I have two, one with a fine grit, the other a medium. I don't want to sound like a GP ad, but they have a centering tool too. It presses into the balsa, creating a center line. Then I use a pencil to highlight the line. Using these two items is a real easy way to taper the rudder and elevator
Where a belt sander is really handy, is cutting down the belly block on the US. It sure saves a lot of cutting and sanding
happy new year
bob