N44DK Paint Cracks Before and During Repair
Return
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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Left trailing horizontal stabilizer |
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