By Udo Rumpf
Allow at least 400 hrs.
If you have a good overall paint adhesion simply sand paint with 80 grit with a small orbital sander, be careful not to break through the primer over the rips, spar and other hard point. After you have laid down the splines with your profile templates, about every 24", paint the spline rail with a black felt marker, that will be your control when you sand.
See picture in original article.
Fill in the area between the splines, one section at the time only, possible two if they are far a part. It takes about an average of three passes of Microballon-epoxy mixture to bring it up to the first height for sanding. Remember at this point you will already be doing some very controlled sanding.
Leave enough material above the spline rail, about .003"( do not break through the black marker).
The mixture is very dry, it should not weigh more then 3.4 lb/Gallon. The complete rework, including paint, will add about 30lb for the two wings. If you do only the top 50% of the wing you can expect a saving of 10 -12 lb. You will need about 7 Gallons of Micro balloon and 2 Gallons of Epoxy. Lots of that is wasted.
Fill in sections in an alternate fashion. For sanding at this point one needs a 2x3 inches, 6 foot long rectangular aluminum tube. Attach 40 grit sanding paper to the center 30 inches. Both ends should have a spacer to of the same thickness to protect the splines from being sanded off. (Reversed sanding paper can be used for this purpose.)
Once all the section have been filled, you can now focus on the remaining .003" and take it down in a controlled fashion (still working with 40 grit sand paper.) From this point on, it is a matter of filling and sanding the imperfection. and then bringing it down to the spline.
Never use to much pressure while sanding, in the later stages you float your sanding block other wise you will imprint the rip and spar pattern especially where the micro balloon transition into nothing . The sanding block becomes progressively smaller, but not smaller then 18". That way your are not liable to put to much pressure on the surface. After painting you may use a 9" block.
The print through problem is ever present. Remember with the right approach one can stay well below the aerodynamic minimumís.
NOTE: Do not waste your time if your not willing to execute with precision. That includes checking out your old wing to make sure that the modification, in fact, will fit. The upper flange of the rear spar U- channel has to be trimmed back to about save rivet hole distance. A recess could be incorporated for the new mylar seals. This recess can be formed by taping the area prior to the application of the filler.