For larger fins, anything over about 2 sq. I find that the pins that come in shirts are just about the right size. I've found two handy tools for sanding big rockets. 3M makes these sponge-like sanding pads. They are great for conforming to the curves of tubes, nose cones, fillets, etc., and make quick work of fillers.
Big rockets call for heavy duty solutions.
Save the belt sander for airfoiling the fins during construction. This will add strength and improve the aerodynamics of the model. Always keep a bottle of rubbing alcohol handy when working with epoxy. Dip your finger in the alcohol and run it along the fillet to smooth out the bumps. Use 30 minute epoxy with microballoons added.