Dear Gordon, thanks for keeping us attached. The video is not that clear...if I am right the cracks are in flight direction, and there are many of them, is n't it? Mostly the quite thin 1e layer of the composite, say an 80 grams 0/90 glasslayer is only affected. The second layer underneath , mostly a 300 grams +/-45 layer is not affected. Can you specify more precise what the case is here?
Note: on composite sailplanes that use Fiberglass, you can have a beautiful glider with no cracks and a perfect airframe/wings if you take good care of it.
Gel Coat is a Polyester resin that has had a white pigment added. But the airframe is made of epoxy resin. The type of Gelcoat that always cracks (eventually) with age is called Vorgolat. This was used on most Schemp Hirth, Schleicher, Stemme and LS sailplanes (before LS went bust). Grob, DG, Slingsby, Glasflugel and early Jonkers, all used a different formulation of polyester, called called Swabalac that is less prone to cracking. Swabalacs main market was GRP showers and baths. When that market switched to Acrylics in 2008, Swabalac Gel coat sadly was discontinued. The gel coat usually sprayed into the mould in manufacture, when it is fully dry, a coat of epoxy resin is brushed on and the Glass/carbon fibre cloth then applied. The Polyester Gel Coat has a different expansion coefficient compared to the epoxy. Plus it can be quite thick (3mm in places in all the corners of of the moulds where the GRP cloth cannot go round a sharp corner) that makes it brittle. Age cracks often crack in the same direction that it was last sanded. If the Gelcoat is well bonded to the GRP, when the Gel coat cracks, these cracks propagate into the brushed layer of epoxy. Every time the wing flexes or there is a large temperature change, the 2 hard edges of the cracked gel/epoxy resin eventually propagate into the first (sometimes deeper) layer of Glassfibre. This layer then has to be removed and replaced (very expensive in labour and usually adds weight) Most new sailplanes have pure T35 Vorgolat, that without the PU coat does indeed still crack just like the original Vorgolat did. But if coated with polyurethane shortly after build (or refinish) appears to not to crack. All gel coats go very brittle below -20 dgress C. Lots of wave flying with rapid changes of temperature and any manoeuvres that cause wing flexing (like descending with the brakes out), will crack all gel coats where they are thickest. (around the brakes boxes and aileron normally). In the marine industry Gel coat has evolved hugely and might have now fixed these problems. But it appears nobody in the aviation world is willing to try anything new, in case it does not work over the long term.
