For smaller planes... blenderm tape works well and comes in several widths
for larger planes Clear monokote also works well just put it on with the surface fully flexed and use one of those small sealing (trim) irons to get it adhered. Best to put it on the underside of the elevators and ailerons and hide it as well as you can on the rudder.
Sealing reduces drag, reduces the possibility of flutter and increases the effectiveness of the surface