Dear forum members,
I always had a question, never having a chance to see in person a properly done
sample of how much you should sand a control surface layup before applying
filler
to the control surfaces.
If you lightly scuff the surface with 80 grit all you get is minute dullness at
the crest of the layup fiber bundles. I would say not even 10-20% scuffed. If
you go more then you are indeed cutting through the fibers. but my understanding
is that anything that is still glossy provides basically zero bonding strength.
So what is that magical point at which you stop sanding and it is sufficient
bonding
for the filer.
BTW, one time I spoke with a gentleman that happened to run the R&D department
at Lancair during the certification of the Columbia 300. He told me they did
extensive
testing on how much to sand before a structural bond. He indicated that
the highest structural strength with bonded components would be achieved by
doing very detailed cleaning and then just using scuff pads to lightly scuff
the surface. The resulting bonded structure was stronger than cleaning and
sanding
with coarse sandpaper until all surfaces are dull. Now, he explained we are
not talking about the bond strength. He explained that what they tested is
for the overall strength of the bonded structure and his explanation is that
using
coarse sandpaper damages the substrate layups and overall you end up with
a weaker structure. Then he added they still ended up scuffing until dull in
production because the FAA would not accept that just using scuff pads and
ending
with a surface that still looks glossy would be OK despite all the testing.
I found the story interesting.
Best regards,
Chris
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