A dead horse with nine lives. My original post was sparked by Greg's
posting about boats and gliders and was written towards that. During
my tenure at Scaled Composites, we never molded a finish (i.e. sprayed
into mold then laid up parts) 1: because of the need to perform
additional external work to the structure 2: hard to control thickness
of coating when using a spray on coating. This variation can distort
fibers in subtle ways possibly changing designed performance
chacteristics of that structure, but also this variation in thickness
in the coating can play havoc on itself. Temperature cycling will
cause this coating to contract and expand. Areas of thicker material
do this more or less than surrounding material of relatively
consistent thickness. This leads to stress which if given a focal
point, over time can materialize as a surface crack or other anomaly.

With gelcoat and other polyester topcoats(or fillers) shrinking about
1% during their cure, one must wait a sufficient amount of time before
sanding. If done to early the finish contracts around the scratches
imposing a force of sufficient magnitude to generate a crack.

Then at Scaled, aircraft where finished in acrylic enamels or
urethanes and today they use primarily urethane, but they still only
sand to 600 grit when they sand. Folks as a crew chief for one of the
high altitude test vehicles I can tell you that it's seven year old
finish (it's age when I left) never showed any ill effects. 66 foot
wing span 65,000 feet designed operating altitude. And we never waxed
any of the aircraft there including this one. I think sanded
structures done incorrectly or cared for improperly will lead to
cracking eventually, but one must objectively identify and observe the
mode of failure. On their own did the sanding scratches exert enough
force to initiate a crack? Or did something else complete the
triangle?


Reuben

Paul Gaines wrote in message ...
I would like to beat this dead horse a little more.

I will check and rephrase my "cross-linking" term. I would like B. =
Lacovara to explain the additional protection afforded at the surface of =
a very shiny, smooth de-molded gel-coat part that is not aggressively =
sanded/polished. I do know that in many cases this cracking/crazing =
follows the exact pattern of the sanding strokes/motions that are =
performed at the factories. Note the cord-wise cracking on LS products. =
They used to do much of the sanding in that direction. They also used =
jitter bug type sanders on the back bones of the fuselages. You can =
observe these "eyelash" shaped cracks all down the surface of the spines =
of many LS gliders. Schleicher's wing control surfaces crack =
span-wise...the direction of sanding. Final grit numbers and direction =
of sanding and follow-up polishing can make a huge difference in surface =
longevity too. This was explained to me an a very large repair facility =
in Germany a number of years ago when they demonstrated span-wise final =
sanding in two stages, followed by a chord-wise direction of the polish =
machine. DG employs this technique. Lacovara's paper confirms that =
surface "roughness" allows faster degradation on several levels. You =
can increase the life of your factory gel-coat finish by sanding the =
surface with 1500-2000 grit paper and polishing ACROSS this sanded =
pattern, increasing the gloss. There are a number of polishes suitable =
for this, including the hard stick polishes used by the factories and =
3M's Finesse it and Perfect it systems, just to name a couple. Follow =
this up with WX seal/block system, and you are well on your way to a =
happier gel-coat surface. =20

Dry out your ship, slick it up, use WX block system once, preferably =
twice a year, NEVER tie it out, and store it in your trailer inside a =
basement or hanger, etc..

P. Gaines=20