Hello again!
This shall be my concluding post on this topic if nothing new surfaces.
The first thing I don't understand is how much time or raw material could be really saved by painting only half of the underside wing? Ailerons and flaps were painted as was the upperside. The downtime because of paint drying (hardening) is the same and painting the whole wing would have taken some 15 minutes more (my guess based on (little) experience).
Second thing I still don't understand is why no other type had this type of camouflage? For example ground angle of Ju 88 is more or less the same as Fw 190 judged from pictures.
Third thing I don't understand is that camouflage (glare prevention) would take preference over aircraft performance. No colour can prevent shadow etc. revealing the form of aircraft shape. Tarpaulins, camo nets, tree branches etc. would do that. I don't see surface colour as the only option of camouflaging an aircraft.
Smoothing the wing surface can reduce the profile drag coefficient to half*. Even if the wing profile drag is only a portion of total drag in the high-speed end (where induced drag decreases) this can make a real difference. I think any novice LW pilot would find use for extra 20-30 km/h (pure guess, someone with real numbers?) or so for the top-speed of their mounts.
Where my hypothesis in on weak ground is that on the underside there would be no need to smooth the surface further than some 10% from leading edge. The only other wing I know which also has this half-chord treatment is the P-51 Mustang. This was also the original trigger for thought together with the 119 D filler mentioned in the Fw 190 documents.
Surfacer - if used - would also be beneficial over the rivet lines on the leading edge (upperside). This is where the difference in smoothness would be really felt. The photos I have looked are not of such quality to see if the rivet heads are covered or not. The ones showing undersides clearly are usually crashed wrecks and panel lines on them are not 100% counter proof against surfacer use.
Surfacer (filler) does not have to be some thick stuff spread with spatula. There are very fast drying nitroputties which can (and could) be shot with sprayguns. I've been involved in couple of repainting projects of glassfiber wings (a glider and a motorplane (twice)) and do know what the sanding entails. I also know that with right tools the sanding can be done quickly and efficiently especially as on the metal wing there are no fiber reinforcements ("sand-throughs") to be worried about.
Thanks for the responses and would love to see more of them.
Regards to all,
Kari
* Ref: NACA report 667 "Determination fo the profile drag of an airplane in flight at high Reynolds numers"
http://naca.larc.nasa.gov/reports/1939/naca-report-667/
Quote from the abstract: "... the surface irregularities on the original wing increased the profile-drag coefficient 50 percent above that of the smooth wing."