On Thursday, September 26, 2019 at 10:24:16 AM UTC-4, Tango Whisky wrote:
Le mardi 24 septembre 2019 23:48:48 UTC+2, Papa3 a écritÂ*:
To expand on Hank's comment, we've looked carefully at a couple of heavily crazed gliders (a Grob left tied out and an LS4 that was not well maintained) under magnification. After removing the failed gelcoat, the crazing lines are still visible in the glass/epoxy. Our resident materials scientist was able to see small voids in the epoxy (a few 10s of microns deep) and few failures of the glass fiber (individual strands) at the surface level. Think of it as very shallow "pitting" of the epoxy in the outermost glass layer. There is no sign of it penetrating deeper than that. I've heard of some shops "painting" a warm coat of epoxy into particularly bad areas and others either peeling off the outer glass layer and replacing. Never saw the need to do either of these.

p3

These small voids on the outmost epoxy layer are absolutely normal and due to the manufacturing method. The outmost glass fiber layer is laminated into the mould which has been spray-coated with the gelcoat. This process will always trap air between this glass layer and the gelcoat (although curing under vacuum does get rid of most of it).

These small voids are main reason that the first step on a sanded-down wing is to laminate a very thin layer of glass fibers. This layer does not help the structure - it efficiently fills up those small voids. Otherwise, any sprayed-on filler or gelcoat won't be able to fill up the voids due to the relatively high surface tension, creating thousands of nasty litte craters on the filler/gelcoat surface.

Bert
Ventus cM "TW"

If the structure is not compromised we fill the "million pinholes" using polyester surface filler applied with a foam roller. This works filler into the voids and avoids the bridging that results if filler is sprayed.
Laminating another layer, without peeling the top ply,can result in undesired shape change, and when sanded smooth, can leave defects that still require detail filling.
FWIW
UH