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How to bind a hairline fiberglass crack w/o access to the back side.

istrumit

Well Known Member
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I have a small fiberglass crack between the windscreen and the cowling....its exactly centered and about 6 inches below the bottom of the wind screen.

You can't see the crack in the attached picture, but I put an arrow to show where it is (you seriously can't see it in the pic).

Its about 4 inches long, hairline, and allows the fiberglass on the back side of the crack (closest to the windscreen) to stick up about 3/32 of an inch.

Its been that way since I bought the airplane....and it is not getting worse.

My A/P suggested that I get a hypodermic needle and inject some lock tite (or other) adhesive under it to bind it back down.

My question is, what adhesive to use ? It has to basically be instantly bonded because I don't have a way to hold it down while it bonds.

And I don't want it to generate heat and melt the surrounding material.

Ideas ?
 
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If your goal is to fill the void while bonding the area, your A&P's suggestion should work. You might also consider covering with wax paper and a sand bag to help with closing the crack but then may have to deal with any adhesive that squeezes out.

I would use 2 part epoxy resin as the adhesive but I imagine there are other options.
 
I'm a fiberglass guy and am scratching my head over your description. If one side of the crack is sticking up 3/32" above the other side, it's not what I would call a "hairline crack". I would need a better photo to see what's going on, but it could be either a structural failure in the fiberglass or just a cosmetic failure in any filler that might have been used. If it really is 3/32" - and I don't see that in the photo - then it should be sanded down, inspected, repaired...and repainted.
 
I'm a fiberglass guy and am scratching my head over your description. If one side of the crack is sticking up 3/32" above the other side, it's not what I would call a "hairline crack". I would need a better photo to see what's going on, but it could be either a structural failure in the fiberglass or just a cosmetic failure in any filler that might have been used. If it really is 3/32" - and I don't see that in the photo - then it should be sanded down, inspected, repaired...and repainted.

I agree with Snopercod on this one. I would want to know WHY it happened before I figured out how to fix it.
 
Be CAREFUL

Be extremely careful using Loctite! Read the info in the particular loctite you are using as it WILL ATTACK some plastics....
 
Henkel Loctite anaerobic threadlocker

When I think of Loctite I think of their anaerobic threadlockers which I'm gonna say won't work They make other products though.

From the Henkle Loctite website: "Anaerobic adhesives remain liquid until isolated from oxygen in the presence of metal ions."
 
had same prob

I'm a fiberglass guy and am scratching my head over your description. If one side of the crack is sticking up 3/32" above the other side, it's not what I would call a "hairline crack". I would need a better photo to see what's going on, but it could be either a structural failure in the fiberglass or just a cosmetic failure in any filler that might have been used. If it really is 3/32" - and I don't see that in the photo - then it should be sanded down, inspected, repaired...and repainted.

Before I was flying I had the same thing happen in the same spot. The skin has some flex there where you blend the windscreen to the skin. I once pressed the spot in the skin while working in the area and got a little crack. The skin flexed but the fairing didn't. In my case it was maybe 3 inches long and otherwise as described.

To fix I mixed up some flox/micro, flexed the skin (on purpose this time), worked in the mixture, released the flexed skin to let it come back to a natural position, let cure, sand, and re-primed the area. My airplane isn't painted yet so it wasn't really a big deal.

That was about a year ago and I haven't had any more cracks but I do find myself more careful when working in that area.
 
Before I was flying I had the same thing happen in the same spot.
To me it sounds like there might be a weakness in that area, but if you weren't flying yet, I don't know. I had a hairline crack in the front part of my lower cowling behind the spinner. I ended up having to apply a couple layers of fiberglass cloth to the back side to strengthen the area.
 
Based on the limited information listed here, I would guess that the fg separated from the aluminum, probably due to a poor mechanical bond in that area. Basically, the aluminum was likely not roughed up enough for the resin to grip
I have seen this a few times and the only correct fix is to grind out and redo that area
Putting a blind rivet or two through the fg and skin at construction helps, in fact I believe VANs instructions shows that method
It?s a bummer but better than having a chunk come off in flight
Just my view of what it might be.
 
Based on the limited information listed here, I would guess that the fg separated from the aluminum, probably due to a poor mechanical bond in that area.
Oh well heck, I didn't realize there was an aluminum-to-fiberglass joint there. I have NEVER had any luck bonding fiberglass to aluminum. The best results seem to be obtained by cleaning and wet sanding the aluminum with 320 grit, then etching, then using structural adhesive to make the bond. The structural adhesive (Like HYSOL EA-9430) has a higher T-peel rating than regular epoxy. Even so, sf3543 had the right idea about using blind rivets.
 
Just as another point of reference.
I think most folks use the West System for their FG work and bond directly to the aluminum skin, or plexiglass.
When I rough up the area to be bonded to, I use at least 80 grit or even 35 to get the best mechanical grip I can. I just do it by hand to get fairly aggressive scratching without completely sanding the area smooth. This has worked well for me, along with the blind rivets in strategic spots.
I have seen a lot of windscreen strips get knocked off due to not roughing up the aluminum, or plexi, enough to get a good bond.
 
Yes, sounds like delamination

I agree. Given the info provided it sounds like a delamination issue between the aluminum skin and the fiberglass fairing which is laid up in place as multiple layers of glass. Ideally you will need to solve for why the separation is occurring, grind away and lay up a patch. As an alternative adding a couple of blind rivets as well as new adhesive under the delaminated area sounds like a reasonable approach.


Based on the limited information listed here, I would guess that the fg separated from the aluminum, probably due to a poor mechanical bond in that area. Basically, the aluminum was likely not roughed up enough for the resin to grip
I have seen this a few times and the only correct fix is to grind out and redo that area
Putting a blind rivet or two through the fg and skin at construction helps, in fact I believe VANs instructions shows that method
It?s a bummer but better than having a chunk come off in flight
Just my view of what it might be.
 
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