If a plane is properly put together, improper sanding of exterior surfaces will not affect flight much. However, it will greatly affect how sleek and refined the craft looks at completion. It is crucial to sand joining surfaces that will be adhered for a better bond, but superficial sanding can be done to any degree of desired perfection.
It helps to have adequate sanding tools and materials. For large surfaces, a large block sander is helpful to ensure that surfaces are flat and consistent. Often a hand planer help start off with a smooth surface. Small curves surfaces can be sanded with various hand or automatic tools, but a skilled hobbyist can often achieve the same or better results with just a piece of sandpaper. Glue joints should be smoothed out to prevent roughness showing through the plane's skin. Larger spurs or glue deposits can also be removed carefully with a hobby knife. Inspecting the plane from many angles and striving for symmetry, smoothness and consistency yields the best results.
Alignment and measurements
Each structural component of an RC plane is usually built separately, then assemble toward the end of construction. The final assembly of these structural parts is an extremely crucial step in making a flyable craft. A slant in the vertical stabilizer and rudder will make the craft turn more sharply to one side than the other. If the trailing edge of the wings are lower than expected, the plane will have more drag in normal flight and will stall more easily. If right and left wings are not mounted symmetrically, various things could occur, including: unwanted roll, drag to one side, turns too sharply on one side due to steep dihedral, etc... The side affects of improperly mounted components are numerous.
A hobbyist must use proper tools and patience when mounting components. Clamps, pins, tape, and even a very small drop of glue can aid in positioning parts before final adhesives are applied. Incidence meters, rulers, tape measures, levels, contractors triangles, protractors and just plain visual detection are important tools in finding structural nuances and imperfections. Components should be inspected and measured from all possible angles before final adhering.