On Tuesday, November 17, 2015 at 8:30:21 AM UTC-6, wrote:
On Monday, November 16, 2015 at 9:53:30 PM UTC-5, wrote:
On Sunday, November 15, 2015 at 6:17:38 AM UTC-7, son_of_flubber wrote:
On Sunday, November 15, 2015 at 4:34:23 AM UTC-5, Paul Crabb wrote:

PU or Acrylic topcoat paint is just used as a very thin UV block, it is usually a very thin layer and is not used to "profile" the surface.

Does the PU protect the gelcoat from temperature related crazing?
Someone told me that gelcoat crazing was attributed to rapid change in temperature associated with flying wave. Once the cracks start moisture penetrates and accelerates the deterioration.

Anyone have a crazed PU painted wing?

Comment- Yep- where painting was done over surfaces where all compromised gelcoat was not removed and surface simply filled.

Anyone have a better explanation for wing crazing?

I've just sanded away the old gelcoat off the wings of my 1978 LS3. I've noticed a couple interesting patterns regarding the gel coat crazing.

a) All crazing stopped abruptly at the edge of any covered portion of the wings gel coat. So for example, the gel coat that was covered by turbulator tape and Mylar tape was perfectly free of any crazing. The crazing lines dead ended at the boundary.

Comment- Over coating, such as number painting, seals the surface and pretty much stops degradation.

What this means to me, is that at least some portion of crazing is due to evaporation/drying/degradation over time of an exposed critical gel coat chemical. I imagine that some organic molecule embedded in the gel-coat composition over time breaks up and likes to go float around in the atmosphere. I know it is not the sun or UV causing breakup because the bottom surfaces of the wing and stab presented the same patterns and the same amount of crazing.

b) All crazing lines followed an underlying surface defect or discontinuity. For example in my wings, stab, and fuse they followed previous sanding patterns perfectly. This makes sense as the deepest sanding gouges would naturally have the greatest density of exposed surface area and thus degrade the fastest. Or so my theory goes. Interestingly enough there was a perfect craze line that matched the exact pattern of the underside original owners initials that where painted on the wing, JB.

Comment- This is why finish sanding needs to be very well done so as to not leave deep scratches which are initiation points for moisture, UV, and mechanical stress.

c) The crazing extended to the contact layer of the fiberglass/epoxy layer leaving visible troughs. This very thin contact layer was a distinct material between the gel coat and the fiberglass and was the yellow color of vanilla ice cream. I have no clue what this material is, although Dave Nelson suggested it could be the remaining layer of a previous gel coat. I also thought it could be the "Warning Colour" material called out in the LS3 Maintenance manual as:
Nitro Cellulose Kombilack Reinorange RAL 2004 made by Lackfabrik Bader on page 13.9.

Comment- This is likely to be a change in the surface characteristics of the gelcoat where the epoxy for laminating was applied wet on wet over the tacky gelcoat. Tis is done to get the best bond between two very different materials.

Despite looking extensively through a jewelers magnifying glass I saw no evidence of the crazing cracks extending into the fiberglass matrix.

The only caveat is that Rolladen Schneider used a sandwich structure for the wings and stab skins on the LS3. Not sure if this is true for other sailplane manufacturers. I noticed the outer stab skin is very very thin and can be easily sanded into if you're not careful. According to the maintenance manual, the outer layer of the sandwich has only two layers.

Comment- Almost all primary surfaces can be expected to be sandwich, except fuselages which are mostly thick single layer.

I'd appreciate if you could comment on my findings/theory. One thing for sure, is that I could not identify a single crazing line on my LS3 that was due to mechanical stress or skin shearing like you typically find on the corners of the spoiler box on a Grob. I have some good pictures I'll post.