The following are "before" pictures of three areas showing cosmetic paint cracking. The trailing inboard roughly 12 inch square portions on both the right and left wings are common areas for cracking on earlier 300Ls. Stepping too far aft on the left wing could explain these cracks but the same exact pattern appears on the right wing. The third picture shows what happens when someone not familiar with Extras pushes on the trailing edge of the horizontal stabilizer to move the plane (like you would on a 172). There is a faring strip attached to the horizontal stabilizer that acts as a gap seal to the elevator. If you push on this, the paint cracks where the faring strip joins the horizontal stabilizer.
The area on the trailing inboard edge of each wing was sanded down to expose the previous repair. That repair consisted of someone removing an approximately 0.050" portion of pink "Bondo" filler that had cracked and refilling it with a more modern and compliant blue body putty filler. The area was then sprayed with high build primer and re-sprayed with white. The crack returned right next to the previous filler repair.
The Extra service manual and current Sickkens/Extra refinish specification specifically precludes the use of high build primers. Only an epoxy primer is allowed in the process. The previous attempt(s) at correcting the paint cracking on the wings was unsuccessful and a review of the root cause helps explain this problem. The first observation the painter made was that there was a very thick layer of body putty in the two wing areas showing cracking. The painter then pointed out that you can look across the wing and see a fairly uniform display of the base composite (carbon) material on most of the inner flat surface of the wing. This is normal as the paint tends to shrink over time. However, once you approach the outer edges of the flat wing surface the finish becomes very smooth with little to no "print through" of the composite material. The trailing inboard edge of both wings is a very good example of this tendency. This is speculation on my part but I believe the first set of molds for the 300L had imperfections in certain areas, especially where a vertical surface meets a horizontal one. Body putty was used to fill these areas and, in the case of the wing root which is very much in the customer's eye, A LOT of filler was used to smooth the area. The molds could have been especially rough in these areas as well adding to the problem. In addition, inexperienced production workers could (and from our observations did) use way too much body putty; more that was required to make the surface flat. Carbon composite skins will flex some and body putty's ability to accommodate flexing without cracking is inversely proportional to its thickness. In short, if too thick it will crack. It is just a matter of time.
In the case of N44DK, we took the body putty down to the gel coat such that only that putty that was necessary to make the area flat was retained. This was very time consuming as there was a lot of putty to remove. The area was then sprayed with the appropriate epoxy primer and was allowed to cure for a few days before being wet sanded and then sprayed with a light coat of white. Any areas where there were cracks were scalloped out (about a half inch in width) and refilled. I have included a picture of both the left and right wing shortly after primer. Pictures of the completed painted wing can be seen on the main page for N44DK.
There is significantly less body filler on the inboard trailing edges of the wings now and epoxy primer has been used to seal the surface. Although not a guarantee that the paint cracks will not return, I believe we have significantly reduced the likelihood of having to revisit this problem. It was also good to finally find and understand the root cause of this cracking and to see that it has nothing to do with the parent composite material.
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