Good questions
Having sanded a few airplanes, the first thing I noticed when I started sanding the sheeting from ZN (my first prep job was on Mr. T. Yamada's Synergy) was that contest weight sheeting requires special care.
First off, any seams and rough work is knocked down with worn 80 grit. Yes, 80. A soft hand will not scrape into the balsa.
IMPORTANT: During final sanding, some compessed air course really helps. The paper must be lifted often and and hard particles blown out to keep from scratching the surface. The ears will tell immediately if there is debris in the paper.
Once all of the shapes are there, 240 is used to remove any remaining bumps. A wood block keeps from sanding dips around any hard spots. Then 400, first with the grain, and then I do the little circles. I use foam as my block because it conforms to the curve of the surface and rounds any spots flattened in the earlier processes.
After the 400 is finished, I fill all imperfections. Sand off the filler, recheck, refill other imperfections, sand off, then I start with the 1000 grit. 1000 grit sands contest grade balsa in about the same way that 400 grit sands heavier grade, albeit leaving the surface with a 1000 grit finish. When the surface is as smooth as possible, I blow it with compressed air (GENTLY, it is possible to blow right through 1.5mm balsa with compressed air). This clears out the grain and shows any nicks that remain. Nicks that look pressed get a licked finger, nick that look a little torn get a spot of filler. (filler is NHP blasa filler; my friend and real professional builder Nathan [he does Christophe's prototypes and advises me when I need help] from ZNLine France used dope and talc).
As far as boundries (LE and wing tips where they join the sheeting), if they show a line, then they get a little filler. Normally, it is only the tips because the grains are 90 degrees and that will be highlighted under the covering after a month. I also fill the deeper grain pathways. The finished product, when held at various angles to the lights, should have an even grained surface with no manmade nicks, no pits or ripples.
I would love to know if there are easier methods that can achieve the same results. I know of no method other than the elbow grease method of sand, clean, look, fill and continue on until the surface looks as perfect as possible. There are so many blocking techniques and most work as well as the other because the hand learns the feel of the tool and the same result can be achieved.
Maybe a separate thread on F3A airplane finishing techniques could be started? Like I wrote earlier, contest weight balsa needs different techniques than harder grades of balsa, so this is special in a way to F3A.
Is all of this work overkill? After the first few flights the wing normally has nicks just from insects or grass or a clumsy hand with dried CA on the fingers. I have heard some highly experienced people give theories on smooth versus rough finishes and that rougher finishes fly better. I have also seen CPLR's personal paper finished wings that look great at 20 feet but up close are obvioulsy optimized for winning in the air, not on the static display. I guess it's like this; we will give the customer the best and lightest finish that we can do using our methods (balsa sheeted, Oracover finished). If the desire for a rougher finish is there, we can supply unsheeted cores with a basic kit for the person who wants to do it all, as there are so many good techniques.
OK, that was long winded. I hope that this answered some questions. I continue to learn and we continue to experiment with new methods, continually striving to find better ways to make better airplanes.
To Dan Reaville,
Thanks for the kind words. I am very happy that you like the airplane and the kit. Your work looks very professional and I shall continue to view your progress and learn from any different ideas that you have. I am as curious as anybody about your mystery, so I hope you finish soon so that I get to see what it is!!!
Regards,
Mark